path: root/synapse/storage/_base.py

# -*- coding: utf-8 -*-
# Copyright 2014, 2015 OpenMarket Ltd
#
# 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

from synapse.api.errors import StoreError
from synapse.events import FrozenEvent
from synapse.events.utils import prune_event
from synapse.util.logutils import log_function
from synapse.util.logcontext import PreserveLoggingContext, LoggingContext
from synapse.util.lrucache import LruCache
import synapse.metrics

from twisted.internet import defer

from collections import namedtuple, OrderedDict
import simplejson as json
import sys
import time


logger = logging.getLogger(__name__)

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


metrics = synapse.metrics.get_metrics_for("synapse.storage")

sql_scheduling_timer = metrics.register_distribution("schedule_time")

sql_query_timer = metrics.register_distribution("query_time", labels=["verb"])
sql_txn_timer = metrics.register_distribution("transaction_time", labels=["desc"])
sql_getevents_timer = metrics.register_distribution("getEvents_time", labels=["desc"])

caches_by_name = {}
cache_counter = metrics.register_cache(
    "cache",
    lambda: {(name,): len(caches_by_name[name]) for name in caches_by_name.keys()},
    labels=["name"],
)


# TODO(paul):
#  * more generic key management
#  * consider other eviction strategies - LRU?
def cached(max_entries=1000):
    """ A method decorator that applies a memoizing cache around the function.

    The function is presumed to take one additional argument, which is used as
    the key for the cache. Cache hits are served directly from the cache;
    misses use the function body to generate the value.

    The wrapped function has an additional member, a callable called
    "invalidate". This can be used to remove individual entries from the cache.

    The wrapped function has another additional callable, called "prefill",
    which can be used to insert values into the cache specifically, without
    calling the calculation function.
    """
    def wrap(orig):
        cache = OrderedDict()
        name = orig.__name__

        caches_by_name[name] = cache

        def prefill(key, value):
            while len(cache) > max_entries:
                cache.popitem(last=False)

            cache[key] = value

        @defer.inlineCallbacks
        def wrapped(self, key):
            if key in cache:
                cache_counter.inc_hits(name)
                defer.returnValue(cache[key])

            cache_counter.inc_misses(name)
            ret = yield orig(self, key)
            prefill(key, ret)
            defer.returnValue(ret)

        def invalidate(key):
            cache.pop(key, None)

        wrapped.invalidate = invalidate
        wrapped.prefill = prefill
        return wrapped

    return wrap


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

    def __init__(self, txn, name):
        object.__setattr__(self, "txn", txn)
        object.__setattr__(self, "name", name)

    def __getattr__(self, name):
        return getattr(self.txn, name)

    def __setattr__(self, name, value):
        setattr(self.txn, name, value)

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

        try:
            if args and args[0]:
                values = args[0]
                sql_logger.debug(
                    "[SQL values] {%s} " + ", ".join(("<%r>",) * len(values)),
                    self.name,
                    *values
                )
        except:
            # Don't let logging failures stop SQL from working
            pass

        start = time.time() * 1000
        try:
            return self.txn.execute(
                sql, *args, **kwargs
            )
        except:
                logger.exception("[SQL FAIL] {%s}", self.name)
                raise
        finally:
            msecs = (time.time() * 1000) - start
            sql_logger.debug("[SQL time] {%s} %f", self.name, msecs)
            sql_query_timer.inc_by(msecs, sql.split()[0])


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

    def update(self, key, start_time, end_time=None):
        if end_time is None:
            end_time = time.time() * 1000
        duration = end_time - start_time
        count, cum_time = self.current_counters.get(key, (0, 0))
        count += 1
        cum_time += duration
        self.current_counters[key] = (count, cum_time)
        return end_time

    def interval(self, interval_duration, limit=3):
        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,
                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


class SQLBaseStore(object):
    _TXN_ID = 0

    def __init__(self, hs):
        self.hs = hs
        self._db_pool = hs.get_db_pool()
        self._clock = hs.get_clock()

        self._previous_txn_total_time = 0
        self._current_txn_total_time = 0
        self._previous_loop_ts = 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._get_event_counters = PerformanceCounters()

        self._get_event_cache = LruCache(hs.config.event_cache_size)

        # Pretend the getEventCache is just another named cache
        caches_by_name["*getEvent*"] = self._get_event_cache

    def start_profiling(self):
        self._previous_loop_ts = self._clock.time_msec()

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

            time_now = self._clock.time_msec()
            time_then = self._previous_loop_ts
            self._previous_loop_ts = time_now

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

            top_three_counters = self._txn_perf_counters.interval(
                time_now - time_then, limit=3
            )

            top_3_event_counters = self._get_event_counters.interval(
                time_now - time_then, limit=3
            )

            logger.info(
                "Total database time: %.3f%% {%s} {%s}",
                ratio * 100, top_three_counters, top_3_event_counters
            )

        self._clock.looping_call(loop, 10000)

    @defer.inlineCallbacks
    def runInteraction(self, desc, func, *args, **kwargs):
        """Wraps the .runInteraction() method on the underlying db_pool."""
        current_context = LoggingContext.current_context()

        start_time = time.time() * 1000

        def inner_func(txn, *args, **kwargs):
            with LoggingContext("runInteraction") as context:
                current_context.copy_to(context)
                start = time.time() * 1000
                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) % (sys.maxint - 1)

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

                sql_scheduling_timer.inc_by(time.time() * 1000 - start_time)
                transaction_logger.debug("[TXN START] {%s}", name)
                try:
                    return func(LoggingTransaction(txn, name), *args, **kwargs)
                except:
                    logger.exception("[TXN FAIL] {%s}", name)
                    raise
                finally:
                    end = time.time() * 1000
                    duration = end - start

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

                    self._current_txn_total_time += duration
                    self._txn_perf_counters.update(desc, start, end)
                    sql_txn_timer.inc_by(duration, desc)

        with PreserveLoggingContext():
            result = yield self._db_pool.runInteraction(
                inner_func, *args, **kwargs
            )
        defer.returnValue(result)

    def cursor_to_dict(self, cursor):
        """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 = list(column[0] for column in cursor.description)
        results = list(
            dict(zip(col_headers, row)) for row in cursor.fetchall()
        )
        return results

    def _execute(self, desc, decoder, query, *args):
        """Runs a single query for a result set.

        Args:
            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):
            cursor = txn.execute(query, args)
            if decoder:
                return decoder(cursor)
            else:
                return cursor.fetchall()

        return self.runInteraction(desc, interaction)

    def _execute_and_decode(self, desc, query, *args):
        return self._execute(desc, self.cursor_to_dict, query, *args)

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

    def _simple_insert(self, table, values, or_replace=False, or_ignore=False):
        """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_replace : bool; if True performs an INSERT OR REPLACE
        """
        return self.runInteraction(
            "_simple_insert",
            self._simple_insert_txn, table, values, or_replace=or_replace,
            or_ignore=or_ignore,
        )

    @log_function
    def _simple_insert_txn(self, txn, table, values, or_replace=False,
                           or_ignore=False):
        sql = "%s INTO %s (%s) VALUES(%s)" % (
            ("INSERT OR REPLACE" if or_replace else
             "INSERT OR IGNORE" if or_ignore else "INSERT"),
            table,
            ", ".join(k for k in values),
            ", ".join("?" for k in values)
        )

        logger.debug(
            "[SQL] %s Args=%s",
            sql, values.values(),
        )

        txn.execute(sql, values.values())
        return txn.lastrowid

    def _simple_upsert(self, table, keyvalues, values):
        """
        Args:
            table (str): The table to upsert into
            keyvalues (dict): The unique key tables and their new values
            values (dict): The nonunique columns and their new values
        Returns: A deferred
        """
        return self.runInteraction(
            "_simple_upsert",
            self._simple_upsert_txn, table, keyvalues, values
        )

    def _simple_upsert_txn(self, txn, table, keyvalues, values):
        # Try to update
        sql = "UPDATE %s SET %s WHERE %s" % (
            table,
            ", ".join("%s = ?" % (k,) for k in values),
            " AND ".join("%s = ?" % (k,) for k in keyvalues)
        )
        sqlargs = values.values() + keyvalues.values()
        logger.debug(
            "[SQL] %s Args=%s",
            sql, sqlargs,
        )

        txn.execute(sql, sqlargs)
        if txn.rowcount == 0:
            # We didn't update and rows so insert a new one
            allvalues = {}
            allvalues.update(keyvalues)
            allvalues.update(values)

            sql = "INSERT INTO %s (%s) VALUES (%s)" % (
                table,
                ", ".join(k for k in allvalues),
                ", ".join("?" for _ in allvalues)
            )
            logger.debug(
                "[SQL] %s Args=%s",
                sql, keyvalues.values(),
            )
            txn.execute(sql, allvalues.values())

    def _simple_select_one(self, table, keyvalues, retcols,
                           allow_none=False):
        """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
            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
        """
        return self._simple_selectupdate_one(
            table, keyvalues, retcols=retcols, allow_none=allow_none
        )

    def _simple_select_one_onecol(self, table, keyvalues, retcol,
                                  allow_none=False):
        """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
        """
        return self.runInteraction(
            "_simple_select_one_onecol",
            self._simple_select_one_onecol_txn,
            table, keyvalues, retcol, allow_none=allow_none,
        )

    def _simple_select_one_onecol_txn(self, txn, table, keyvalues, retcol,
                                      allow_none=False):
        ret = self._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")

    def _simple_select_onecol_txn(self, txn, table, keyvalues, retcol):
        sql = (
            "SELECT %(retcol)s FROM %(table)s WHERE %(where)s "
            "ORDER BY rowid asc"
        ) % {
            "retcol": retcol,
            "table": table,
            "where": " AND ".join("%s = ?" % k for k in keyvalues.keys()),
        }

        txn.execute(sql, keyvalues.values())

        return [r[0] for r in txn.fetchall()]

    def _simple_select_onecol(self, table, keyvalues, retcol):
        """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 (str): table name
            keyvalues (dict): column names and values to select the rows with
            retcol (str): column whos value we wish to retrieve.

        Returns:
            Deferred: Results in a list
        """
        return self.runInteraction(
            "_simple_select_onecol",
            self._simple_select_onecol_txn,
            table, keyvalues, retcol
        )

    def _simple_select_list(self, table, keyvalues, retcols):
        """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 : string giving the table name
            keyvalues : dict of column names and values to select the rows with,
            or None to not apply a WHERE clause.
            retcols : list of strings giving the names of the columns to return
        """
        return self.runInteraction(
            "_simple_select_list",
            self._simple_select_list_txn,
            table, keyvalues, retcols
        )

    def _simple_select_list_txn(self, txn, table, keyvalues, retcols):
        """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 : string giving the table name
            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 keyvalues:
            sql = "SELECT %s FROM %s WHERE %s ORDER BY rowid asc" % (
                ", ".join(retcols),
                table,
                " AND ".join("%s = ?" % (k, ) for k in keyvalues)
            )
            txn.execute(sql, keyvalues.values())
        else:
            sql = "SELECT %s FROM %s ORDER BY rowid asc" % (
                ", ".join(retcols),
                table
            )
            txn.execute(sql)

        return self.cursor_to_dict(txn)

    def _simple_update_one(self, table, keyvalues, updatevalues,
                           retcols=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
            retcols : optional list of column names to return

        If present, retcols gives a list of column names on which to perform
        a SELECT statement *before* performing the UPDATE statement. The values
        of these will be returned in a dict.

        These are performed within the same transaction, allowing an atomic
        get-and-set.  This can be used to implement compare-and-set by putting
        the update column in the 'keyvalues' dict as well.
        """
        return self._simple_selectupdate_one(table, keyvalues, updatevalues,
                                             retcols=retcols)

    def _simple_selectupdate_one(self, table, keyvalues, updatevalues=None,
                                 retcols=None, allow_none=False):
        """ Combined SELECT then UPDATE."""
        if retcols:
            select_sql = "SELECT %s FROM %s WHERE %s ORDER BY rowid asc" % (
                ", ".join(retcols),
                table,
                " AND ".join("%s = ?" % (k) for k in keyvalues)
            )

        if updatevalues:
            update_sql = "UPDATE %s SET %s WHERE %s" % (
                table,
                ", ".join("%s = ?" % (k,) for k in updatevalues),
                " AND ".join("%s = ?" % (k,) for k in keyvalues)
            )

        def func(txn):
            ret = None
            if retcols:
                txn.execute(select_sql, keyvalues.values())

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

                ret = dict(zip(retcols, row))

            if updatevalues:
                txn.execute(
                    update_sql,
                    updatevalues.values() + keyvalues.values()
                )

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

            return ret
        return self.runInteraction("_simple_selectupdate_one", func)

    def _simple_delete_one(self, table, keyvalues):
        """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)
        )

        def func(txn):
            txn.execute(sql, keyvalues.values())
            if txn.rowcount == 0:
                raise StoreError(404, "No row found")
            if txn.rowcount > 1:
                raise StoreError(500, "more than one row matched")
        return self.runInteraction("_simple_delete_one", func)

    def _simple_delete(self, table, keyvalues):
        """Executes a DELETE query on the named table.

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

        return self.runInteraction("_simple_delete", self._simple_delete_txn)

    def _simple_delete_txn(self, txn, table, keyvalues):
        sql = "DELETE FROM %s WHERE %s" % (
            table,
            " AND ".join("%s = ?" % (k, ) for k in keyvalues)
        )

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

    def _simple_max_id(self, table):
        """Executes a SELECT query on the named table, expecting to return the
        max value for the column "id".

        Args:
            table : string giving the table name
        """
        sql = "SELECT MAX(id) AS id FROM %s" % table

        def func(txn):
            txn.execute(sql)
            max_id = self.cursor_to_dict(txn)[0]["id"]
            if max_id is None:
                return 0
            return max_id

        return self.runInteraction("_simple_max_id", func)

    def _get_events(self, event_ids, check_redacted=True,
                    get_prev_content=False):
        return self.runInteraction(
            "_get_events", self._get_events_txn, event_ids,
            check_redacted=check_redacted, get_prev_content=get_prev_content,
        )

    def _get_events_txn(self, txn, event_ids, check_redacted=True,
                        get_prev_content=False):
        if not event_ids:
            return []

        events = [
            self._get_event_txn(
                txn, event_id,
                check_redacted=check_redacted,
                get_prev_content=get_prev_content
            )
            for event_id in event_ids
        ]

        return [e for e in events if e]

    def _get_event_txn(self, txn, event_id, check_redacted=True,
                       get_prev_content=False, allow_rejected=False):

        start_time = time.time() * 1000

        def update_counter(desc, last_time):
            curr_time = self._get_event_counters.update(desc, last_time)
            sql_getevents_timer.inc_by(curr_time - last_time, desc)
            return curr_time

        cache = self._get_event_cache.setdefault(event_id, {})

        try:
            # Separate cache entries for each way to invoke _get_event_txn
            ret = cache[(check_redacted, get_prev_content, allow_rejected)]

            cache_counter.inc_hits("*getEvent*")
            return ret
        except KeyError:
            cache_counter.inc_misses("*getEvent*")
            pass
        finally:
            start_time = update_counter("event_cache", start_time)

        sql = (
            "SELECT e.internal_metadata, e.json, r.event_id, rej.reason "
            "FROM event_json as e "
            "LEFT JOIN redactions as r ON e.event_id = r.redacts "
            "LEFT JOIN rejections as rej on rej.event_id = e.event_id  "
            "WHERE e.event_id = ? "
            "LIMIT 1 "
        )

        txn.execute(sql, (event_id,))

        res = txn.fetchone()

        if not res:
            return None

        internal_metadata, js, redacted, rejected_reason = res

        start_time = update_counter("select_event", start_time)

        if allow_rejected or not rejected_reason:
            result = self._get_event_from_row_txn(
                txn, internal_metadata, js, redacted,
                check_redacted=check_redacted,
                get_prev_content=get_prev_content,
            )
            cache[(check_redacted, get_prev_content, allow_rejected)] = result
            return result
        else:
            return None

    def _get_event_from_row_txn(self, txn, internal_metadata, js, redacted,
                                check_redacted=True, get_prev_content=False):

        start_time = time.time() * 1000

        def update_counter(desc, last_time):
            curr_time = self._get_event_counters.update(desc, last_time)
            sql_getevents_timer.inc_by(curr_time - last_time, desc)
            return curr_time

        d = json.loads(js)
        start_time = update_counter("decode_json", start_time)

        internal_metadata = json.loads(internal_metadata)
        start_time = update_counter("decode_internal", start_time)

        ev = FrozenEvent(d, internal_metadata_dict=internal_metadata)
        start_time = update_counter("build_frozen_event", start_time)

        if check_redacted and redacted:
            ev = prune_event(ev)

            ev.unsigned["redacted_by"] = redacted
            # Get the redaction event.

            because = self._get_event_txn(
                txn,
                redacted,
                check_redacted=False
            )

            if because:
                ev.unsigned["redacted_because"] = because
            start_time = update_counter("redact_event", start_time)

        if get_prev_content and "replaces_state" in ev.unsigned:
            prev = self._get_event_txn(
                txn,
                ev.unsigned["replaces_state"],
                get_prev_content=False,
            )
            if prev:
                ev.unsigned["prev_content"] = prev.get_dict()["content"]
            start_time = update_counter("get_prev_content", start_time)

        return ev

    def _parse_events(self, rows):
        return self.runInteraction(
            "_parse_events", self._parse_events_txn, rows
        )

    def _parse_events_txn(self, txn, rows):
        event_ids = [r["event_id"] for r in rows]

        return self._get_events_txn(txn, event_ids)

    def _has_been_redacted_txn(self, txn, event):
        sql = "SELECT event_id FROM redactions WHERE redacts = ?"
        txn.execute(sql, (event.event_id,))
        result = txn.fetchone()
        return result[0] if result else None


class Table(object):
    """ A base class used to store information about a particular table.
    """

    table_name = None
    """ str: The name of the table """

    fields = None
    """ list: The field names """

    EntryType = None
    """ Type: A tuple type used to decode the results """

    _select_where_clause = "SELECT %s FROM %s WHERE %s"
    _select_clause = "SELECT %s FROM %s"
    _insert_clause = "INSERT OR REPLACE INTO %s (%s) VALUES (%s)"

    @classmethod
    def select_statement(cls, where_clause=None):
        """
        Args:
            where_clause (str): The WHERE clause to use.

        Returns:
            str: An SQL statement to select rows from the table with the given
            WHERE clause.
        """
        if where_clause:
            return cls._select_where_clause % (
                ", ".join(cls.fields),
                cls.table_name,
                where_clause
            )
        else:
            return cls._select_clause % (
                ", ".join(cls.fields),
                cls.table_name,
            )

    @classmethod
    def insert_statement(cls):
        return cls._insert_clause % (
            cls.table_name,
            ", ".join(cls.fields),
            ", ".join(["?"] * len(cls.fields)),
        )

    @classmethod
    def decode_single_result(cls, results):
        """ Given an iterable of tuples, return a single instance of
            `EntryType` or None if the iterable is empty
        Args:
            results (list): The results list to convert to `EntryType`
        Returns:
            EntryType: An instance of `EntryType`
        """
        results = list(results)
        if results:
            return cls.EntryType(*results[0])
        else:
            return None

    @classmethod
    def decode_results(cls, results):
        """ Given an iterable of tuples, return a list of `EntryType`
        Args:
            results (list): The results list to convert to `EntryType`

        Returns:
            list: A list of `EntryType`
        """
        return [cls.EntryType(*row) for row in results]

    @classmethod
    def get_fields_string(cls, prefix=None):
        if prefix:
            to_join = ("%s.%s" % (prefix, f) for f in cls.fields)
        else:
            to_join = cls.fields

        return ", ".join(to_join)


class JoinHelper(object):
    """ Used to help do joins on tables by looking at the tables' fields and
    creating a list of unique fields to use with SELECTs and a namedtuple
    to dump the results into.

    Attributes:
        tables (list): List of `Table` classes
        EntryType (type)
    """

    def __init__(self, *tables):
        self.tables = tables

        res = []
        for table in self.tables:
            res += [f for f in table.fields if f not in res]

        self.EntryType = namedtuple("JoinHelperEntry", res)

    def get_fields(self, **prefixes):
        """Get a string representing a list of fields for use in SELECT
        statements with the given prefixes applied to each.

        For example::

            JoinHelper(PdusTable, StateTable).get_fields(
                PdusTable="pdus",
                StateTable="state"
            )
        """
        res = []
        for field in self.EntryType._fields:
            for table in self.tables:
                if field in table.fields:
                    res.append("%s.%s" % (prefixes[table.__name__], field))
                    break

        return ", ".join(res)

    def decode_results(self, rows):
        return [self.EntryType(*row) for row in rows]