path: root/synapse/storage/database.py

# -*- coding: utf-8 -*-
# Copyright 2014-2016 OpenMarket Ltd
# Copyright 2017-2018 New Vector Ltd
# Copyright 2019 The Matrix.org Foundation C.I.C.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import logging
import time
from sys import intern
from time import monotonic as monotonic_time
from typing import (
    Any,
    Callable,
    Dict,
    Iterable,
    Iterator,
    List,
    Optional,
    Tuple,
    TypeVar,
    overload,
)

from prometheus_client import Histogram
from typing_extensions import Literal

from twisted.enterprise import adbapi
from twisted.internet import defer

from synapse.api.errors import StoreError
from synapse.config.database import DatabaseConnectionConfig
from synapse.logging.context import (
    LoggingContext,
    LoggingContextOrSentinel,
    current_context,
    make_deferred_yieldable,
)
from synapse.metrics.background_process_metrics import run_as_background_process
from synapse.storage.background_updates import BackgroundUpdater
from synapse.storage.engines import BaseDatabaseEngine, PostgresEngine, Sqlite3Engine
from synapse.storage.types import Connection, Cursor
from synapse.types import Collection

# python 3 does not have a maximum int value
MAX_TXN_ID = 2 ** 63 - 1

logger = logging.getLogger(__name__)

sql_logger = logging.getLogger("synapse.storage.SQL")
transaction_logger = logging.getLogger("synapse.storage.txn")
perf_logger = logging.getLogger("synapse.storage.TIME")

sql_scheduling_timer = Histogram("synapse_storage_schedule_time", "sec")

sql_query_timer = Histogram("synapse_storage_query_time", "sec", ["verb"])
sql_txn_timer = Histogram("synapse_storage_transaction_time", "sec", ["desc"])


# Unique indexes which have been added in background updates. Maps from table name
# to the name of the background update which added the unique index to that table.
#
# This is used by the upsert logic to figure out which tables are safe to do a proper
# UPSERT on: until the relevant background update has completed, we
# have to emulate an upsert by locking the table.
#
UNIQUE_INDEX_BACKGROUND_UPDATES = {
    "user_ips": "user_ips_device_unique_index",
    "device_lists_remote_extremeties": "device_lists_remote_extremeties_unique_idx",
    "device_lists_remote_cache": "device_lists_remote_cache_unique_idx",
    "event_search": "event_search_event_id_idx",
}


def make_pool(
    reactor, db_config: DatabaseConnectionConfig, engine: BaseDatabaseEngine
) -> adbapi.ConnectionPool:
    """Get the connection pool for the database.
    """

    return adbapi.ConnectionPool(
        db_config.config["name"],
        cp_reactor=reactor,
        cp_openfun=engine.on_new_connection,
        **db_config.config.get("args", {})
    )


def make_conn(
    db_config: DatabaseConnectionConfig, engine: BaseDatabaseEngine
) -> Connection:
    """Make a new connection to the database and return it.

    Returns:
        Connection
    """

    db_params = {
        k: v
        for k, v in db_config.config.get("args", {}).items()
        if not k.startswith("cp_")
    }
    db_conn = engine.module.connect(**db_params)
    engine.on_new_connection(db_conn)
    return db_conn


# The type of entry which goes on our after_callbacks and exception_callbacks lists.
#
# Python 3.5.2 doesn't support Callable with an ellipsis, so we wrap it in quotes so
# that mypy sees the type but the runtime python doesn't.
_CallbackListEntry = Tuple["Callable[..., None]", Iterable[Any], Dict[str, Any]]


class LoggingTransaction:
    """An object that almost-transparently proxies for the 'txn' object
    passed to the constructor. Adds logging and metrics to the .execute()
    method.

    Args:
        txn: The database transaction object to wrap.
        name: The name of this transactions for logging.
        database_engine
        after_callbacks: A list that callbacks will be appended to
            that have been added by `call_after` which should be run on
            successful completion of the transaction. None indicates that no
            callbacks should be allowed to be scheduled to run.
        exception_callbacks: A list that callbacks will be appended
            to that have been added by `call_on_exception` which should be run
            if transaction ends with an error. None indicates that no callbacks
            should be allowed to be scheduled to run.
    """

    __slots__ = [
        "txn",
        "name",
        "database_engine",
        "after_callbacks",
        "exception_callbacks",
    ]

    def __init__(
        self,
        txn: Cursor,
        name: str,
        database_engine: BaseDatabaseEngine,
        after_callbacks: Optional[List[_CallbackListEntry]] = None,
        exception_callbacks: Optional[List[_CallbackListEntry]] = None,
    ):
        self.txn = txn
        self.name = name
        self.database_engine = database_engine
        self.after_callbacks = after_callbacks
        self.exception_callbacks = exception_callbacks

    def call_after(self, callback: "Callable[..., None]", *args: Any, **kwargs: Any):
        """Call the given callback on the main twisted thread after the
        transaction has finished. Used to invalidate the caches on the
        correct thread.
        """
        # if self.after_callbacks is None, that means that whatever constructed the
        # LoggingTransaction isn't expecting there to be any callbacks; assert that
        # is not the case.
        assert self.after_callbacks is not None
        self.after_callbacks.append((callback, args, kwargs))

    def call_on_exception(
        self, callback: "Callable[..., None]", *args: Any, **kwargs: Any
    ):
        # if self.exception_callbacks is None, that means that whatever constructed the
        # LoggingTransaction isn't expecting there to be any callbacks; assert that
        # is not the case.
        assert self.exception_callbacks is not None
        self.exception_callbacks.append((callback, args, kwargs))

    def fetchall(self) -> List[Tuple]:
        return self.txn.fetchall()

    def fetchone(self) -> Tuple:
        return self.txn.fetchone()

    def __iter__(self) -> Iterator[Tuple]:
        return self.txn.__iter__()

    @property
    def rowcount(self) -> int:
        return self.txn.rowcount

    @property
    def description(self) -> Any:
        return self.txn.description

    def execute_batch(self, sql: str, args: Iterable[Iterable[Any]]) -> None:
        if isinstance(self.database_engine, PostgresEngine):
            from psycopg2.extras import execute_batch  # type: ignore

            self._do_execute(lambda *x: execute_batch(self.txn, *x), sql, args)
        else:
            for val in args:
                self.execute(sql, val)

    def execute(self, sql: str, *args: Any) -> None:
        self._do_execute(self.txn.execute, sql, *args)

    def executemany(self, sql: str, *args: Any) -> None:
        self._do_execute(self.txn.executemany, sql, *args)

    def _make_sql_one_line(self, sql: str) -> str:
        "Strip newlines out of SQL so that the loggers in the DB are on one line"
        return " ".join(line.strip() for line in sql.splitlines() if line.strip())

    def _do_execute(self, func, sql: str, *args: Any) -> None:
        sql = self._make_sql_one_line(sql)

        # TODO(paul): Maybe use 'info' and 'debug' for values?
        sql_logger.debug("[SQL] {%s} %s", self.name, sql)

        sql = self.database_engine.convert_param_style(sql)
        if args:
            try:
                sql_logger.debug("[SQL values] {%s} %r", self.name, args[0])
            except Exception:
                # Don't let logging failures stop SQL from working
                pass

        start = time.time()

        try:
            return func(sql, *args)
        except Exception as e:
            sql_logger.debug("[SQL FAIL] {%s} %s", self.name, e)
            raise
        finally:
            secs = time.time() - start
            sql_logger.debug("[SQL time] {%s} %f sec", self.name, secs)
            sql_query_timer.labels(sql.split()[0]).observe(secs)

    def close(self) -> None:
        self.txn.close()


class PerformanceCounters(object):
    def __init__(self):
        self.current_counters = {}
        self.previous_counters = {}

    def update(self, key: str, duration_secs: float) -> None:
        count, cum_time = self.current_counters.get(key, (0, 0))
        count += 1
        cum_time += duration_secs
        self.current_counters[key] = (count, cum_time)

    def interval(self, interval_duration_secs: float, limit: int = 3) -> str:
        counters = []
        for name, (count, cum_time) in self.current_counters.items():
            prev_count, prev_time = self.previous_counters.get(name, (0, 0))
            counters.append(
                (
                    (cum_time - prev_time) / interval_duration_secs,
                    count - prev_count,
                    name,
                )
            )

        self.previous_counters = dict(self.current_counters)

        counters.sort(reverse=True)

        top_n_counters = ", ".join(
            "%s(%d): %.3f%%" % (name, count, 100 * ratio)
            for ratio, count, name in counters[:limit]
        )

        return top_n_counters


R = TypeVar("R")


class DatabasePool(object):
    """Wraps a single physical database and connection pool.

    A single database may be used by multiple data stores.
    """

    _TXN_ID = 0

    def __init__(
        self, hs, database_config: DatabaseConnectionConfig, engine: BaseDatabaseEngine
    ):
        self.hs = hs
        self._clock = hs.get_clock()
        self._database_config = database_config
        self._db_pool = make_pool(hs.get_reactor(), database_config, engine)

        self.updates = BackgroundUpdater(hs, self)

        self._previous_txn_total_time = 0.0
        self._current_txn_total_time = 0.0
        self._previous_loop_ts = 0.0

        # TODO(paul): These can eventually be removed once the metrics code
        #   is running in mainline, and we have some nice monitoring frontends
        #   to watch it
        self._txn_perf_counters = PerformanceCounters()

        self.engine = engine

        # A set of tables that are not safe to use native upserts in.
        self._unsafe_to_upsert_tables = set(UNIQUE_INDEX_BACKGROUND_UPDATES.keys())

        # We add the user_directory_search table to the blacklist on SQLite
        # because the existing search table does not have an index, making it
        # unsafe to use native upserts.
        if isinstance(self.engine, Sqlite3Engine):
            self._unsafe_to_upsert_tables.add("user_directory_search")

        if self.engine.can_native_upsert:
            # Check ASAP (and then later, every 1s) to see if we have finished
            # background updates of tables that aren't safe to update.
            self._clock.call_later(
                0.0,
                run_as_background_process,
                "upsert_safety_check",
                self._check_safe_to_upsert,
            )

    def is_running(self) -> bool:
        """Is the database pool currently running
        """
        return self._db_pool.running

    async def _check_safe_to_upsert(self) -> None:
        """
        Is it safe to use native UPSERT?

        If there are background updates, we will need to wait, as they may be
        the addition of indexes that set the UNIQUE constraint that we require.

        If the background updates have not completed, wait 15 sec and check again.
        """
        updates = await self.simple_select_list(
            "background_updates",
            keyvalues=None,
            retcols=["update_name"],
            desc="check_background_updates",
        )
        updates = [x["update_name"] for x in updates]

        for table, update_name in UNIQUE_INDEX_BACKGROUND_UPDATES.items():
            if update_name not in updates:
                logger.debug("Now safe to upsert in %s", table)
                self._unsafe_to_upsert_tables.discard(table)

        # If there's any updates still running, reschedule to run.
        if updates:
            self._clock.call_later(
                15.0,
                run_as_background_process,
                "upsert_safety_check",
                self._check_safe_to_upsert,
            )

    def start_profiling(self) -> None:
        self._previous_loop_ts = monotonic_time()

        def loop():
            curr = self._current_txn_total_time
            prev = self._previous_txn_total_time
            self._previous_txn_total_time = curr

            time_now = monotonic_time()
            time_then = self._previous_loop_ts
            self._previous_loop_ts = time_now

            duration = time_now - time_then
            ratio = (curr - prev) / duration

            top_three_counters = self._txn_perf_counters.interval(duration, limit=3)

            perf_logger.debug(
                "Total database time: %.3f%% {%s}", ratio * 100, top_three_counters
            )

        self._clock.looping_call(loop, 10000)

    def new_transaction(
        self,
        conn: Connection,
        desc: str,
        after_callbacks: List[_CallbackListEntry],
        exception_callbacks: List[_CallbackListEntry],
        func: "Callable[..., R]",
        *args: Any,
        **kwargs: Any
    ) -> R:
        start = monotonic_time()
        txn_id = self._TXN_ID

        # We don't really need these to be unique, so lets stop it from
        # growing really large.
        self._TXN_ID = (self._TXN_ID + 1) % (MAX_TXN_ID)

        name = "%s-%x" % (desc, txn_id)

        transaction_logger.debug("[TXN START] {%s}", name)

        try:
            i = 0
            N = 5
            while True:
                cursor = LoggingTransaction(
                    conn.cursor(),
                    name,
                    self.engine,
                    after_callbacks,
                    exception_callbacks,
                )
                try:
                    r = func(cursor, *args, **kwargs)
                    conn.commit()
                    return r
                except self.engine.module.OperationalError as e:
                    # This can happen if the database disappears mid
                    # transaction.
                    transaction_logger.warning(
                        "[TXN OPERROR] {%s} %s %d/%d", name, e, i, N,
                    )
                    if i < N:
                        i += 1
                        try:
                            conn.rollback()
                        except self.engine.module.Error as e1:
                            transaction_logger.warning("[TXN EROLL] {%s} %s", name, e1)
                        continue
                    raise
                except self.engine.module.DatabaseError as e:
                    if self.engine.is_deadlock(e):
                        transaction_logger.warning(
                            "[TXN DEADLOCK] {%s} %d/%d", name, i, N
                        )
                        if i < N:
                            i += 1
                            try:
                                conn.rollback()
                            except self.engine.module.Error as e1:
                                transaction_logger.warning(
                                    "[TXN EROLL] {%s} %s", name, e1,
                                )
                            continue
                    raise
                finally:
                    # we're either about to retry with a new cursor, or we're about to
                    # release the connection. Once we release the connection, it could
                    # get used for another query, which might do a conn.rollback().
                    #
                    # In the latter case, even though that probably wouldn't affect the
                    # results of this transaction, python's sqlite will reset all
                    # statements on the connection [1], which will make our cursor
                    # invalid [2].
                    #
                    # In any case, continuing to read rows after commit()ing seems
                    # dubious from the PoV of ACID transactional semantics
                    # (sqlite explicitly says that once you commit, you may see rows
                    # from subsequent updates.)
                    #
                    # In psycopg2, cursors are essentially a client-side fabrication -
                    # all the data is transferred to the client side when the statement
                    # finishes executing - so in theory we could go on streaming results
                    # from the cursor, but attempting to do so would make us
                    # incompatible with sqlite, so let's make sure we're not doing that
                    # by closing the cursor.
                    #
                    # (*named* cursors in psycopg2 are different and are proper server-
                    # side things, but (a) we don't use them and (b) they are implicitly
                    # closed by ending the transaction anyway.)
                    #
                    # In short, if we haven't finished with the cursor yet, that's a
                    # problem waiting to bite us.
                    #
                    # TL;DR: we're done with the cursor, so we can close it.
                    #
                    # [1]: https://github.com/python/cpython/blob/v3.8.0/Modules/_sqlite/connection.c#L465
                    # [2]: https://github.com/python/cpython/blob/v3.8.0/Modules/_sqlite/cursor.c#L236
                    cursor.close()
        except Exception as e:
            transaction_logger.debug("[TXN FAIL] {%s} %s", name, e)
            raise
        finally:
            end = monotonic_time()
            duration = end - start

            current_context().add_database_transaction(duration)

            transaction_logger.debug("[TXN END] {%s} %f sec", name, duration)

            self._current_txn_total_time += duration
            self._txn_perf_counters.update(desc, duration)
            sql_txn_timer.labels(desc).observe(duration)

    @defer.inlineCallbacks
    def runInteraction(self, desc: str, func: Callable, *args: Any, **kwargs: Any):
        """Starts a transaction on the database and runs a given function

        Arguments:
            desc: description of the transaction, for logging and metrics
            func: callback function, which will be called with a
                database transaction (twisted.enterprise.adbapi.Transaction) as
                its first argument, followed by `args` and `kwargs`.

            args: positional args to pass to `func`
            kwargs: named args to pass to `func`

        Returns:
            Deferred: The result of func
        """
        after_callbacks = []  # type: List[_CallbackListEntry]
        exception_callbacks = []  # type: List[_CallbackListEntry]

        if not current_context():
            logger.warning("Starting db txn '%s' from sentinel context", desc)

        try:
            result = yield defer.ensureDeferred(
                self.runWithConnection(
                    self.new_transaction,
                    desc,
                    after_callbacks,
                    exception_callbacks,
                    func,
                    *args,
                    **kwargs
                )
            )

            for after_callback, after_args, after_kwargs in after_callbacks:
                after_callback(*after_args, **after_kwargs)
        except:  # noqa: E722, as we reraise the exception this is fine.
            for after_callback, after_args, after_kwargs in exception_callbacks:
                after_callback(*after_args, **after_kwargs)
            raise

        return result

    async def runWithConnection(
        self, func: "Callable[..., R]", *args: Any, **kwargs: Any
    ) -> R:
        """Wraps the .runWithConnection() method on the underlying db_pool.

        Arguments:
            func: callback function, which will be called with a
                database connection (twisted.enterprise.adbapi.Connection) as
                its first argument, followed by `args` and `kwargs`.
            args: positional args to pass to `func`
            kwargs: named args to pass to `func`

        Returns:
            The result of func
        """
        parent_context = current_context()  # type: Optional[LoggingContextOrSentinel]
        if not parent_context:
            logger.warning(
                "Starting db connection from sentinel context: metrics will be lost"
            )
            parent_context = None

        start_time = monotonic_time()

        def inner_func(conn, *args, **kwargs):
            with LoggingContext("runWithConnection", parent_context) as context:
                sched_duration_sec = monotonic_time() - start_time
                sql_scheduling_timer.observe(sched_duration_sec)
                context.add_database_scheduled(sched_duration_sec)

                if self.engine.is_connection_closed(conn):
                    logger.debug("Reconnecting closed database connection")
                    conn.reconnect()

                return func(conn, *args, **kwargs)

        return await make_deferred_yieldable(
            self._db_pool.runWithConnection(inner_func, *args, **kwargs)
        )

    @staticmethod
    def cursor_to_dict(cursor: Cursor) -> List[Dict[str, Any]]:
        """Converts a SQL cursor into an list of dicts.

        Args:
            cursor: The DBAPI cursor which has executed a query.
        Returns:
            A list of dicts where the key is the column header.
        """
        col_headers = [intern(str(column[0])) for column in cursor.description]
        results = [dict(zip(col_headers, row)) for row in cursor]
        return results

    @overload
    async def execute(
        self, desc: str, decoder: Literal[None], query: str, *args: Any
    ) -> List[Tuple[Any, ...]]:
        ...

    @overload
    async def execute(
        self, desc: str, decoder: Callable[[Cursor], R], query: str, *args: Any
    ) -> R:
        ...

    async def execute(
        self,
        desc: str,
        decoder: Optional[Callable[[Cursor], R]],
        query: str,
        *args: Any
    ) -> R:
        """Runs a single query for a result set.

        Args:
            desc: description of the transaction, for logging and metrics
            decoder - The function which can resolve the cursor results to
                something meaningful.
            query - The query string to execute
            *args - Query args.
        Returns:
            The result of decoder(results)
        """

        def interaction(txn):
            txn.execute(query, args)
            if decoder:
                return decoder(txn)
            else:
                return txn.fetchall()

        return await self.runInteraction(desc, interaction)

    # "Simple" SQL API methods that operate on a single table with no JOINs,
    # no complex WHERE clauses, just a dict of values for columns.

    async def simple_insert(
        self,
        table: str,
        values: Dict[str, Any],
        or_ignore: bool = False,
        desc: str = "simple_insert",
    ) -> bool:
        """Executes an INSERT query on the named table.

        Args:
            table: string giving the table name
            values: dict of new column names and values for them
            or_ignore: bool stating whether an exception should be raised
                when a conflicting row already exists. If True, False will be
                returned by the function instead
            desc: description of the transaction, for logging and metrics

        Returns:
             Whether the row was inserted or not. Only useful when `or_ignore` is True
        """
        try:
            await self.runInteraction(desc, self.simple_insert_txn, table, values)
        except self.engine.module.IntegrityError:
            # We have to do or_ignore flag at this layer, since we can't reuse
            # a cursor after we receive an error from the db.
            if not or_ignore:
                raise
            return False
        return True

    @staticmethod
    def simple_insert_txn(
        txn: LoggingTransaction, table: str, values: Dict[str, Any]
    ) -> None:
        keys, vals = zip(*values.items())

        sql = "INSERT INTO %s (%s) VALUES(%s)" % (
            table,
            ", ".join(k for k in keys),
            ", ".join("?" for _ in keys),
        )

        txn.execute(sql, vals)

    async def simple_insert_many(
        self, table: str, values: List[Dict[str, Any]], desc: str
    ) -> None:
        """Executes an INSERT query on the named table.

        Args:
            table: string giving the table name
            values: dict of new column names and values for them
            desc: description of the transaction, for logging and metrics
        """
        await self.runInteraction(desc, self.simple_insert_many_txn, table, values)

    @staticmethod
    def simple_insert_many_txn(
        txn: LoggingTransaction, table: str, values: List[Dict[str, Any]]
    ) -> None:
        """Executes an INSERT query on the named table.

        Args:
            txn: The transaction to use.
            table: string giving the table name
            values: dict of new column names and values for them
        """
        if not values:
            return

        # This is a *slight* abomination to get a list of tuples of key names
        # and a list of tuples of value names.
        #
        # i.e. [{"a": 1, "b": 2}, {"c": 3, "d": 4}]
        #         => [("a", "b",), ("c", "d",)] and [(1, 2,), (3, 4,)]
        #
        # The sort is to ensure that we don't rely on dictionary iteration
        # order.
        keys, vals = zip(
            *[zip(*(sorted(i.items(), key=lambda kv: kv[0]))) for i in values if i]
        )

        for k in keys:
            if k != keys[0]:
                raise RuntimeError("All items must have the same keys")

        sql = "INSERT INTO %s (%s) VALUES(%s)" % (
            table,
            ", ".join(k for k in keys[0]),
            ", ".join("?" for _ in keys[0]),
        )

        txn.executemany(sql, vals)

    async def simple_upsert(
        self,
        table: str,
        keyvalues: Dict[str, Any],
        values: Dict[str, Any],
        insertion_values: Dict[str, Any] = {},
        desc: str = "simple_upsert",
        lock: bool = True,
    ) -> Optional[bool]:
        """

        `lock` should generally be set to True (the default), but can be set
        to False if either of the following are true:

        * there is a UNIQUE INDEX on the key columns. In this case a conflict
          will cause an IntegrityError in which case this function will retry
          the update.

        * we somehow know that we are the only thread which will be updating
          this table.

        Args:
            table: The table to upsert into
            keyvalues: The unique key columns and their new values
            values: The nonunique columns and their new values
            insertion_values: additional key/values to use only when inserting
            desc: description of the transaction, for logging and metrics
            lock: True to lock the table when doing the upsert.
        Returns:
            Native upserts always return None. Emulated upserts return True if a
            new entry was created, False if an existing one was updated.
        """
        attempts = 0
        while True:
            try:
                return await self.runInteraction(
                    desc,
                    self.simple_upsert_txn,
                    table,
                    keyvalues,
                    values,
                    insertion_values,
                    lock=lock,
                )
            except self.engine.module.IntegrityError as e:
                attempts += 1
                if attempts >= 5:
                    # don't retry forever, because things other than races
                    # can cause IntegrityErrors
                    raise

                # presumably we raced with another transaction: let's retry.
                logger.warning(
                    "IntegrityError when upserting into %s; retrying: %s", table, e
                )

    def simple_upsert_txn(
        self,
        txn: LoggingTransaction,
        table: str,
        keyvalues: Dict[str, Any],
        values: Dict[str, Any],
        insertion_values: Dict[str, Any] = {},
        lock: bool = True,
    ) -> Optional[bool]:
        """
        Pick the UPSERT method which works best on the platform. Either the
        native one (Pg9.5+, recent SQLites), or fall back to an emulated method.

        Args:
            txn: The transaction to use.
            table: The table to upsert into
            keyvalues: The unique key tables and their new values
            values: The nonunique columns and their new values
            insertion_values: additional key/values to use only when inserting
            lock: True to lock the table when doing the upsert.
        Returns:
            Native upserts always return None. Emulated upserts return True if a
            new entry was created, False if an existing one was updated.
        """
        if self.engine.can_native_upsert and table not in self._unsafe_to_upsert_tables:
            self.simple_upsert_txn_native_upsert(
                txn, table, keyvalues, values, insertion_values=insertion_values
            )
            return None
        else:
            return self.simple_upsert_txn_emulated(
                txn,
                table,
                keyvalues,
                values,
                insertion_values=insertion_values,
                lock=lock,
            )

    def simple_upsert_txn_emulated(
        self,
        txn: LoggingTransaction,
        table: str,
        keyvalues: Dict[str, Any],
        values: Dict[str, Any],
        insertion_values: Dict[str, Any] = {},
        lock: bool = True,
    ) -> bool:
        """
        Args:
            table: The table to upsert into
            keyvalues: The unique key tables and their new values
            values: The nonunique columns and their new values
            insertion_values: additional key/values to use only when inserting
            lock: True to lock the table when doing the upsert.
        Returns:
            Returns True if a new entry was created, False if an existing
            one was updated.
        """
        # We need to lock the table :(, unless we're *really* careful
        if lock:
            self.engine.lock_table(txn, table)

        def _getwhere(key):
            # If the value we're passing in is None (aka NULL), we need to use
            # IS, not =, as NULL = NULL equals NULL (False).
            if keyvalues[key] is None:
                return "%s IS ?" % (key,)
            else:
                return "%s = ?" % (key,)

        if not values:
            # If `values` is empty, then all of the values we care about are in
            # the unique key, so there is nothing to UPDATE. We can just do a
            # SELECT instead to see if it exists.
            sql = "SELECT 1 FROM %s WHERE %s" % (
                table,
                " AND ".join(_getwhere(k) for k in keyvalues),
            )
            sqlargs = list(keyvalues.values())
            txn.execute(sql, sqlargs)
            if txn.fetchall():
                # We have an existing record.
                return False
        else:
            # First try to update.
            sql = "UPDATE %s SET %s WHERE %s" % (
                table,
                ", ".join("%s = ?" % (k,) for k in values),
                " AND ".join(_getwhere(k) for k in keyvalues),
            )
            sqlargs = list(values.values()) + list(keyvalues.values())

            txn.execute(sql, sqlargs)
            if txn.rowcount > 0:
                # successfully updated at least one row.
                return False

        # We didn't find any existing rows, so insert a new one
        allvalues = {}  # type: Dict[str, Any]
        allvalues.update(keyvalues)
        allvalues.update(values)
        allvalues.update(insertion_values)

        sql = "INSERT INTO %s (%s) VALUES (%s)" % (
            table,
            ", ".join(k for k in allvalues),
            ", ".join("?" for _ in allvalues),
        )
        txn.execute(sql, list(allvalues.values()))
        # successfully inserted
        return True

    def simple_upsert_txn_native_upsert(
        self,
        txn: LoggingTransaction,
        table: str,
        keyvalues: Dict[str, Any],
        values: Dict[str, Any],
        insertion_values: Dict[str, Any] = {},
    ) -> None:
        """
        Use the native UPSERT functionality in recent PostgreSQL versions.

        Args:
            table: The table to upsert into
            keyvalues: The unique key tables and their new values
            values: The nonunique columns and their new values
            insertion_values: additional key/values to use only when inserting
        """
        allvalues = {}  # type: Dict[str, Any]
        allvalues.update(keyvalues)
        allvalues.update(insertion_values)

        if not values:
            latter = "NOTHING"
        else:
            allvalues.update(values)
            latter = "UPDATE SET " + ", ".join(k + "=EXCLUDED." + k for k in values)

        sql = ("INSERT INTO %s (%s) VALUES (%s) ON CONFLICT (%s) DO %s") % (
            table,
            ", ".join(k for k in allvalues),
            ", ".join("?" for _ in allvalues),
            ", ".join(k for k in keyvalues),
            latter,
        )
        txn.execute(sql, list(allvalues.values()))

    def simple_upsert_many_txn(
        self,
        txn: LoggingTransaction,
        table: str,
        key_names: Collection[str],
        key_values: Collection[Iterable[Any]],
        value_names: Collection[str],
        value_values: Iterable[Iterable[str]],
    ) -> None:
        """
        Upsert, many times.

        Args:
            table: The table to upsert into
            key_names: The key column names.
            key_values: A list of each row's key column values.
            value_names: The value column names
            value_values: A list of each row's value column values.
                Ignored if value_names is empty.
        """
        if self.engine.can_native_upsert and table not in self._unsafe_to_upsert_tables:
            return self.simple_upsert_many_txn_native_upsert(
                txn, table, key_names, key_values, value_names, value_values
            )
        else:
            return self.simple_upsert_many_txn_emulated(
                txn, table, key_names, key_values, value_names, value_values
            )

    def simple_upsert_many_txn_emulated(
        self,
        txn: LoggingTransaction,
        table: str,
        key_names: Iterable[str],
        key_values: Collection[Iterable[Any]],
        value_names: Collection[str],
        value_values: Iterable[Iterable[str]],
    ) -> None:
        """
        Upsert, many times, but without native UPSERT support or batching.

        Args:
            table: The table to upsert into
            key_names: The key column names.
            key_values: A list of each row's key column values.
            value_names: The value column names
            value_values: A list of each row's value column values.
                Ignored if value_names is empty.
        """
        # No value columns, therefore make a blank list so that the following
        # zip() works correctly.
        if not value_names:
            value_values = [() for x in range(len(key_values))]

        for keyv, valv in zip(key_values, value_values):
            _keys = {x: y for x, y in zip(key_names, keyv)}
            _vals = {x: y for x, y in zip(value_names, valv)}

            self.simple_upsert_txn_emulated(txn, table, _keys, _vals)

    def simple_upsert_many_txn_native_upsert(
        self,
        txn: LoggingTransaction,
        table: str,
        key_names: Collection[str],
        key_values: Collection[Iterable[Any]],
        value_names: Collection[str],
        value_values: Iterable[Iterable[Any]],
    ) -> None:
        """
        Upsert, many times, using batching where possible.

        Args:
            table: The table to upsert into
            key_names: The key column names.
            key_values: A list of each row's key column values.
            value_names: The value column names
            value_values: A list of each row's value column values.
                Ignored if value_names is empty.
        """
        allnames = []  # type: List[str]
        allnames.extend(key_names)
        allnames.extend(value_names)

        if not value_names:
            # No value columns, therefore make a blank list so that the
            # following zip() works correctly.
            latter = "NOTHING"
            value_values = [() for x in range(len(key_values))]
        else:
            latter = "UPDATE SET " + ", ".join(
                k + "=EXCLUDED." + k for k in value_names
            )

        sql = "INSERT INTO %s (%s) VALUES (%s) ON CONFLICT (%s) DO %s" % (
            table,
            ", ".join(k for k in allnames),
            ", ".join("?" for _ in allnames),
            ", ".join(key_names),
            latter,
        )

        args = []

        for x, y in zip(key_values, value_values):
            args.append(tuple(x) + tuple(y))

        return txn.execute_batch(sql, args)

    @overload
    async def simple_select_one(
        self,
        table: str,
        keyvalues: Dict[str, Any],
        retcols: Iterable[str],
        allow_none: Literal[False] = False,
        desc: str = "simple_select_one",
    ) -> Dict[str, Any]:
        ...

    @overload
    async def simple_select_one(
        self,
        table: str,
        keyvalues: Dict[str, Any],
        retcols: Iterable[str],
        allow_none: Literal[True] = True,
        desc: str = "simple_select_one",
    ) -> Optional[Dict[str, Any]]:
        ...

    async def simple_select_one(
        self,
        table: str,
        keyvalues: Dict[str, Any],
        retcols: Iterable[str],
        allow_none: bool = False,
        desc: str = "simple_select_one",
    ) -> Optional[Dict[str, Any]]:
        """Executes a SELECT query on the named table, which is expected to
        return a single row, returning multiple columns from it.

        Args:
            table: string giving the table name
            keyvalues: dict of column names and values to select the row with
            retcols: list of strings giving the names of the columns to return
            allow_none: If true, return None instead of failing if the SELECT
                statement returns no rows
            desc: description of the transaction, for logging and metrics
        """
        return await self.runInteraction(
            desc, self.simple_select_one_txn, table, keyvalues, retcols, allow_none
        )

    @overload
    async def simple_select_one_onecol(
        self,
        table: str,
        keyvalues: Dict[str, Any],
        retcol: Iterable[str],
        allow_none: Literal[False] = False,
        desc: str = "simple_select_one_onecol",
    ) -> Any:
        ...

    @overload
    async def simple_select_one_onecol(
        self,
        table: str,
        keyvalues: Dict[str, Any],
        retcol: Iterable[str],
        allow_none: Literal[True] = True,
        desc: str = "simple_select_one_onecol",
    ) -> Optional[Any]:
        ...

    async def simple_select_one_onecol(
        self,
        table: str,
        keyvalues: Dict[str, Any],
        retcol: Iterable[str],
        allow_none: bool = False,
        desc: str = "simple_select_one_onecol",
    ) -> Optional[Any]:
        """Executes a SELECT query on the named table, which is expected to
        return a single row, returning a single column from it.

        Args:
            table: string giving the table name
            keyvalues: dict of column names and values to select the row with
            retcol: string giving the name of the column to return
            allow_none: If true, return None instead of failing if the SELECT
                statement returns no rows
            desc: description of the transaction, for logging and metrics
        """
        return await self.runInteraction(
            desc,
            self.simple_select_one_onecol_txn,
            table,
            keyvalues,
            retcol,
            allow_none=allow_none,
        )

    @classmethod
    def simple_select_one_onecol_txn(
        cls,
        txn: LoggingTransaction,
        table: str,
        keyvalues: Dict[str, Any],
        retcol: Iterable[str],
        allow_none: bool = False,
    ) -> Optional[Any]:
        ret = cls.simple_select_onecol_txn(
            txn, table=table, keyvalues=keyvalues, retcol=retcol
        )

        if ret:
            return ret[0]
        else:
            if allow_none:
                return None
            else:
                raise StoreError(404, "No row found")

    @staticmethod
    def simple_select_onecol_txn(
        txn: LoggingTransaction,
        table: str,
        keyvalues: Dict[str, Any],
        retcol: Iterable[str],
    ) -> List[Any]:
        sql = ("SELECT %(retcol)s FROM %(table)s") % {"retcol": retcol, "table": table}

        if keyvalues:
            sql += " WHERE %s" % " AND ".join("%s = ?" % k for k in keyvalues.keys())
            txn.execute(sql, list(keyvalues.values()))
        else:
            txn.execute(sql)

        return [r[0] for r in txn]

    async def simple_select_onecol(
        self,
        table: str,
        keyvalues: Optional[Dict[str, Any]],
        retcol: str,
        desc: str = "simple_select_onecol",
    ) -> List[Any]:
        """Executes a SELECT query on the named table, which returns a list
        comprising of the values of the named column from the selected rows.

        Args:
            table: table name
            keyvalues: column names and values to select the rows with
            retcol: column whos value we wish to retrieve.
            desc: description of the transaction, for logging and metrics

        Returns:
            Results in a list
        """
        return await self.runInteraction(
            desc, self.simple_select_onecol_txn, table, keyvalues, retcol
        )

    async def simple_select_list(
        self,
        table: str,
        keyvalues: Optional[Dict[str, Any]],
        retcols: Iterable[str],
        desc: str = "simple_select_list",
    ) -> List[Dict[str, Any]]:
        """Executes a SELECT query on the named table, which may return zero or
        more rows, returning the result as a list of dicts.

        Args:
            table: the table name
            keyvalues:
                column names and values to select the rows with, or None to not
                apply a WHERE clause.
            retcols: the names of the columns to return
            desc: description of the transaction, for logging and metrics

        Returns:
            A list of dictionaries.
        """
        return await self.runInteraction(
            desc, self.simple_select_list_txn, table, keyvalues, retcols
        )

    @classmethod
    def simple_select_list_txn(
        cls,
        txn: LoggingTransaction,
        table: str,
        keyvalues: Optional[Dict[str, Any]],
        retcols: Iterable[str],
    ) -> List[Dict[str, Any]]:
        """Executes a SELECT query on the named table, which may return zero or
        more rows, returning the result as a list of dicts.

        Args:
            txn: Transaction object
            table: the table name
            keyvalues:
                column names and values to select the rows with, or None to not
                apply a WHERE clause.
            retcols: the names of the columns to return
        """
        if keyvalues:
            sql = "SELECT %s FROM %s WHERE %s" % (
                ", ".join(retcols),
                table,
                " AND ".join("%s = ?" % (k,) for k in keyvalues),
            )
            txn.execute(sql, list(keyvalues.values()))
        else:
            sql = "SELECT %s FROM %s" % (", ".join(retcols), table)
            txn.execute(sql)

        return cls.cursor_to_dict(txn)

    async def simple_select_many_batch(
        self,
        table: str,
        column: str,
        iterable: Iterable[Any],
        retcols: Iterable[str],
        keyvalues: Dict[str, Any] = {},
        desc: str = "simple_select_many_batch",
        batch_size: int = 100,
    ) -> List[Any]:
        """Executes a SELECT query on the named table, which may return zero or
        more rows, returning the result as a list of dicts.

        Filters rows by whether the value of `column` is in `iterable`.

        Args:
            table: string giving the table name
            column: column name to test for inclusion against `iterable`
            iterable: list
            retcols: list of strings giving the names of the columns to return
            keyvalues: dict of column names and values to select the rows with
            desc: description of the transaction, for logging and metrics
            batch_size: the number of rows for each select query
        """
        results = []  # type: List[Dict[str, Any]]

        if not iterable:
            return results

        # iterables can not be sliced, so convert it to a list first
        it_list = list(iterable)

        chunks = [
            it_list[i : i + batch_size] for i in range(0, len(it_list), batch_size)
        ]
        for chunk in chunks:
            rows = await self.runInteraction(
                desc,
                self.simple_select_many_txn,
                table,
                column,
                chunk,
                keyvalues,
                retcols,
            )

            results.extend(rows)

        return results

    @classmethod
    def simple_select_many_txn(
        cls,
        txn: LoggingTransaction,
        table: str,
        column: str,
        iterable: Iterable[Any],
        keyvalues: Dict[str, Any],
        retcols: Iterable[str],
    ) -> List[Dict[str, Any]]:
        """Executes a SELECT query on the named table, which may return zero or
        more rows, returning the result as a list of dicts.

        Filters rows by whether the value of `column` is in `iterable`.

        Args:
            txn: Transaction object
            table: string giving the table name
            column: column name to test for inclusion against `iterable`
            iterable: list
            keyvalues: dict of column names and values to select the rows with
            retcols: list of strings giving the names of the columns to return
        """
        if not iterable:
            return []

        clause, values = make_in_list_sql_clause(txn.database_engine, column, iterable)
        clauses = [clause]

        for key, value in keyvalues.items():
            clauses.append("%s = ?" % (key,))
            values.append(value)

        sql = "SELECT %s FROM %s WHERE %s" % (
            ", ".join(retcols),
            table,
            " AND ".join(clauses),
        )

        txn.execute(sql, values)
        return cls.cursor_to_dict(txn)

    async def simple_update(
        self,
        table: str,
        keyvalues: Dict[str, Any],
        updatevalues: Dict[str, Any],
        desc: str,
    ) -> int:
        return await self.runInteraction(
            desc, self.simple_update_txn, table, keyvalues, updatevalues
        )

    @staticmethod
    def simple_update_txn(
        txn: LoggingTransaction,
        table: str,
        keyvalues: Dict[str, Any],
        updatevalues: Dict[str, Any],
    ) -> int:
        if keyvalues:
            where = "WHERE %s" % " AND ".join("%s = ?" % k for k in keyvalues.keys())
        else:
            where = ""

        update_sql = "UPDATE %s SET %s %s" % (
            table,
            ", ".join("%s = ?" % (k,) for k in updatevalues),
            where,
        )

        txn.execute(update_sql, list(updatevalues.values()) + list(keyvalues.values()))

        return txn.rowcount

    async def simple_update_one(
        self,
        table: str,
        keyvalues: Dict[str, Any],
        updatevalues: Dict[str, Any],
        desc: str = "simple_update_one",
    ) -> None:
        """Executes an UPDATE query on the named table, setting new values for
        columns in a row matching the key values.

        Args:
            table: string giving the table name
            keyvalues: dict of column names and values to select the row with
            updatevalues: dict giving column names and values to update
            desc: description of the transaction, for logging and metrics
        """
        await self.runInteraction(
            desc, self.simple_update_one_txn, table, keyvalues, updatevalues
        )

    @classmethod
    def simple_update_one_txn(
        cls,
        txn: LoggingTransaction,
        table: str,
        keyvalues: Dict[str, Any],
        updatevalues: Dict[str, Any],
    ) -> None:
        rowcount = cls.simple_update_txn(txn, table, keyvalues, updatevalues)

        if rowcount == 0:
            raise StoreError(404, "No row found (%s)" % (table,))
        if rowcount > 1:
            raise StoreError(500, "More than one row matched (%s)" % (table,))

    # Ideally we could use the overload decorator here to specify that the
    # return type is only optional if allow_none is True, but this does not work
    # when you call a static method from an instance.
    # See https://github.com/python/mypy/issues/7781
    @staticmethod
    def simple_select_one_txn(
        txn: LoggingTransaction,
        table: str,
        keyvalues: Dict[str, Any],
        retcols: Iterable[str],
        allow_none: bool = False,
    ) -> Optional[Dict[str, Any]]:
        select_sql = "SELECT %s FROM %s WHERE %s" % (
            ", ".join(retcols),
            table,
            " AND ".join("%s = ?" % (k,) for k in keyvalues),
        )

        txn.execute(select_sql, list(keyvalues.values()))
        row = txn.fetchone()

        if not row:
            if allow_none:
                return None
            raise StoreError(404, "No row found (%s)" % (table,))
        if txn.rowcount > 1:
            raise StoreError(500, "More than one row matched (%s)" % (table,))

        return dict(zip(retcols, row))

    async def simple_delete_one(
        self, table: str, keyvalues: Dict[str, Any], desc: str = "simple_delete_one"
    ) -> None:
        """Executes a DELETE query on the named table, expecting to delete a
        single row.

        Args:
            table: string giving the table name
            keyvalues: dict of column names and values to select the row with
            desc: description of the transaction, for logging and metrics
        """
        await self.runInteraction(desc, self.simple_delete_one_txn, table, keyvalues)

    @staticmethod
    def simple_delete_one_txn(
        txn: LoggingTransaction, table: str, keyvalues: Dict[str, Any]
    ) -> None:
        """Executes a DELETE query on the named table, expecting to delete a
        single row.

        Args:
            table: string giving the table name
            keyvalues: dict of column names and values to select the row with
        """
        sql = "DELETE FROM %s WHERE %s" % (
            table,
            " AND ".join("%s = ?" % (k,) for k in keyvalues),
        )

        txn.execute(sql, list(keyvalues.values()))
        if txn.rowcount == 0:
            raise StoreError(404, "No row found (%s)" % (table,))
        if txn.rowcount > 1:
            raise StoreError(500, "More than one row matched (%s)" % (table,))

    async def simple_delete(
        self, table: str, keyvalues: Dict[str, Any], desc: str
    ) -> int:
        """Executes a DELETE query on the named table.

        Filters rows by the key-value pairs.

        Args:
            table: string giving the table name
            keyvalues: dict of column names and values to select the row with
            desc: description of the transaction, for logging and metrics

        Returns:
            The number of deleted rows.
        """
        return await self.runInteraction(desc, self.simple_delete_txn, table, keyvalues)

    @staticmethod
    def simple_delete_txn(
        txn: LoggingTransaction, table: str, keyvalues: Dict[str, Any]
    ) -> int:
        """Executes a DELETE query on the named table.

        Filters rows by the key-value pairs.

        Args:
            table: string giving the table name
            keyvalues: dict of column names and values to select the row with

        Returns:
            The number of deleted rows.
        """
        sql = "DELETE FROM %s WHERE %s" % (
            table,
            " AND ".join("%s = ?" % (k,) for k in keyvalues),
        )

        txn.execute(sql, list(keyvalues.values()))
        return txn.rowcount

    async def simple_delete_many(
        self,
        table: str,
        column: str,
        iterable: Iterable[Any],
        keyvalues: Dict[str, Any],
        desc: str,
    ) -> int:
        """Executes a DELETE query on the named table.

        Filters rows by if value of `column` is in `iterable`.

        Args:
            table: string giving the table name
            column: column name to test for inclusion against `iterable`
            iterable: list
            keyvalues: dict of column names and values to select the rows with
            desc: description of the transaction, for logging and metrics

        Returns:
            Number rows deleted
        """
        return await self.runInteraction(
            desc, self.simple_delete_many_txn, table, column, iterable, keyvalues
        )

    @staticmethod
    def simple_delete_many_txn(
        txn: LoggingTransaction,
        table: str,
        column: str,
        iterable: Iterable[Any],
        keyvalues: Dict[str, Any],
    ) -> int:
        """Executes a DELETE query on the named table.

        Filters rows by if value of `column` is in `iterable`.

        Args:
            txn: Transaction object
            table: string giving the table name
            column: column name to test for inclusion against `iterable`
            iterable: list
            keyvalues: dict of column names and values to select the rows with

        Returns:
            Number rows deleted
        """
        if not iterable:
            return 0

        sql = "DELETE FROM %s" % table

        clause, values = make_in_list_sql_clause(txn.database_engine, column, iterable)
        clauses = [clause]

        for key, value in keyvalues.items():
            clauses.append("%s = ?" % (key,))
            values.append(value)

        if clauses:
            sql = "%s WHERE %s" % (sql, " AND ".join(clauses))
        txn.execute(sql, values)

        return txn.rowcount

    def get_cache_dict(
        self,
        db_conn: Connection,
        table: str,
        entity_column: str,
        stream_column: str,
        max_value: int,
        limit: int = 100000,
    ) -> Tuple[Dict[Any, int], int]:
        # Fetch a mapping of room_id -> max stream position for "recent" rooms.
        # It doesn't really matter how many we get, the StreamChangeCache will
        # do the right thing to ensure it respects the max size of cache.
        sql = (
            "SELECT %(entity)s, MAX(%(stream)s) FROM %(table)s"
            " WHERE %(stream)s > ? - %(limit)s"
            " GROUP BY %(entity)s"
        ) % {
            "table": table,
            "entity": entity_column,
            "stream": stream_column,
            "limit": limit,
        }

        sql = self.engine.convert_param_style(sql)

        txn = db_conn.cursor()
        txn.execute(sql, (int(max_value),))

        cache = {row[0]: int(row[1]) for row in txn}

        txn.close()

        if cache:
            min_val = min(cache.values())
        else:
            min_val = max_value

        return cache, min_val

    @classmethod
    def simple_select_list_paginate_txn(
        cls,
        txn: LoggingTransaction,
        table: str,
        orderby: str,
        start: int,
        limit: int,
        retcols: Iterable[str],
        filters: Optional[Dict[str, Any]] = None,
        keyvalues: Optional[Dict[str, Any]] = None,
        order_direction: str = "ASC",
    ) -> List[Dict[str, Any]]:
        """
        Executes a SELECT query on the named table with start and limit,
        of row numbers, which may return zero or number of rows from start to limit,
        returning the result as a list of dicts.

        Use `filters` to search attributes using SQL wildcards and/or `keyvalues` to
        select attributes with exact matches. All constraints are joined together
        using 'AND'.

        Args:
            txn: Transaction object
            table: the table name
            orderby: Column to order the results by.
            start: Index to begin the query at.
            limit: Number of results to return.
            retcols: the names of the columns to return
            filters:
                column names and values to filter the rows with, or None to not
                apply a WHERE ? LIKE ? clause.
            keyvalues:
                column names and values to select the rows with, or None to not
                apply a WHERE clause.
            order_direction: Whether the results should be ordered "ASC" or "DESC".

        Returns:
            The result as a list of dictionaries.
        """
        if order_direction not in ["ASC", "DESC"]:
            raise ValueError("order_direction must be one of 'ASC' or 'DESC'.")

        where_clause = "WHERE " if filters or keyvalues else ""
        arg_list = []  # type: List[Any]
        if filters:
            where_clause += " AND ".join("%s LIKE ?" % (k,) for k in filters)
            arg_list += list(filters.values())
        where_clause += " AND " if filters and keyvalues else ""
        if keyvalues:
            where_clause += " AND ".join("%s = ?" % (k,) for k in keyvalues)
            arg_list += list(keyvalues.values())

        sql = "SELECT %s FROM %s %s ORDER BY %s %s LIMIT ? OFFSET ?" % (
            ", ".join(retcols),
            table,
            where_clause,
            orderby,
            order_direction,
        )
        txn.execute(sql, arg_list + [limit, start])

        return cls.cursor_to_dict(txn)

    async def simple_search_list(
        self,
        table: str,
        term: Optional[str],
        col: str,
        retcols: Iterable[str],
        desc="simple_search_list",
    ) -> Optional[List[Dict[str, Any]]]:
        """Executes a SELECT query on the named table, which may return zero or
        more rows, returning the result as a list of dicts.

        Args:
            table: the table name
            term: term for searching the table matched to a column.
            col: column to query term should be matched to
            retcols: the names of the columns to return

        Returns:
            A list of dictionaries or None.
        """

        return await self.runInteraction(
            desc, self.simple_search_list_txn, table, term, col, retcols
        )

    @classmethod
    def simple_search_list_txn(
        cls,
        txn: LoggingTransaction,
        table: str,
        term: Optional[str],
        col: str,
        retcols: Iterable[str],
    ) -> Optional[List[Dict[str, Any]]]:
        """Executes a SELECT query on the named table, which may return zero or
        more rows, returning the result as a list of dicts.

        Args:
            txn: Transaction object
            table: the table name
            term: term for searching the table matched to a column.
            col: column to query term should be matched to
            retcols: the names of the columns to return

        Returns:
            None if no term is given, otherwise a list of dictionaries.
        """
        if term:
            sql = "SELECT %s FROM %s WHERE %s LIKE ?" % (", ".join(retcols), table, col)
            termvalues = ["%%" + term + "%%"]
            txn.execute(sql, termvalues)
        else:
            return None

        return cls.cursor_to_dict(txn)


def make_in_list_sql_clause(
    database_engine: BaseDatabaseEngine, column: str, iterable: Iterable
) -> Tuple[str, list]:
    """Returns an SQL clause that checks the given column is in the iterable.

    On SQLite this expands to `column IN (?, ?, ...)`, whereas on Postgres
    it expands to `column = ANY(?)`. While both DBs support the `IN` form,
    using the `ANY` form on postgres means that it views queries with
    different length iterables as the same, helping the query stats.

    Args:
        database_engine
        column: Name of the column
        iterable: The values to check the column against.

    Returns:
        A tuple of SQL query and the args
    """

    if database_engine.supports_using_any_list:
        # This should hopefully be faster, but also makes postgres query
        # stats easier to understand.
        return "%s = ANY(?)" % (column,), [list(iterable)]
    else:
        return "%s IN (%s)" % (column, ",".join("?" for _ in iterable)), list(iterable)


KV = TypeVar("KV")


def make_tuple_comparison_clause(
    database_engine: BaseDatabaseEngine, keys: List[Tuple[str, KV]]
) -> Tuple[str, List[KV]]:
    """Returns a tuple comparison SQL clause

    Depending what the SQL engine supports, builds a SQL clause that looks like either
    "(a, b) > (?, ?)", or "(a > ?) OR (a == ? AND b > ?)".

    Args:
        database_engine
        keys: A set of (column, value) pairs to be compared.

    Returns:
        A tuple of SQL query and the args
    """
    if database_engine.supports_tuple_comparison:
        return (
            "(%s) > (%s)" % (",".join(k[0] for k in keys), ",".join("?" for _ in keys)),
            [k[1] for k in keys],
        )

    # we want to build a clause
    #    (a > ?) OR
    #    (a == ? AND b > ?) OR
    #    (a == ? AND b == ? AND c > ?)
    #    ...
    #    (a == ? AND b == ? AND ... AND z > ?)
    #
    # or, equivalently:
    #
    #  (a > ? OR (a == ? AND
    #    (b > ? OR (b == ? AND
    #      ...
    #        (y > ? OR (y == ? AND
    #          z > ?
    #        ))
    #      ...
    #    ))
    #  ))
    #
    # which itself is equivalent to (and apparently easier for the query optimiser):
    #
    #  (a >= ? AND (a > ? OR
    #    (b >= ? AND (b > ? OR
    #      ...
    #        (y >= ? AND (y > ? OR
    #          z > ?
    #        ))
    #      ...
    #    ))
    #  ))
    #
    #

    clause = ""
    args = []  # type: List[KV]
    for k, v in keys[:-1]:
        clause = clause + "(%s >= ? AND (%s > ? OR " % (k, k)
        args.extend([v, v])

    (k, v) = keys[-1]
    clause += "%s > ?" % (k,)
    args.append(v)

    clause += "))" * (len(keys) - 1)
    return clause, args