1 files changed, 87 insertions, 7 deletions
diff --git a/synapse/storage/database.py b/synapse/storage/database.py
index e61595336c..a7cd97b0b0 100644
--- a/synapse/storage/database.py
+++ b/synapse/storage/database.py
@@ -17,7 +17,17 @@
 import logging
 import time
 from time import monotonic as monotonic_time
-from typing import Any, Callable, Dict, Iterable, Iterator, List, Optional, Tuple
+from typing import (
+    Any,
+    Callable,
+    Dict,
+    Iterable,
+    Iterator,
+    List,
+    Optional,
+    Tuple,
+    TypeVar,
+)
 
 from six import iteritems, iterkeys, itervalues
 from six.moves import intern, range
@@ -32,6 +42,7 @@ 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
@@ -483,7 +494,7 @@ class Database(object):
             end = monotonic_time()
             duration = end - start
 
-            LoggingContext.current_context().add_database_transaction(duration)
+            current_context().add_database_transaction(duration)
 
             transaction_logger.debug("[TXN END] {%s} %f sec", name, duration)
 
@@ -510,7 +521,7 @@ class Database(object):
         after_callbacks = []  # type: List[_CallbackListEntry]
         exception_callbacks = []  # type: List[_CallbackListEntry]
 
-        if LoggingContext.current_context() == LoggingContext.sentinel:
+        if not current_context():
             logger.warning("Starting db txn '%s' from sentinel context", desc)
 
         try:
@@ -547,10 +558,8 @@ class Database(object):
         Returns:
             Deferred: The result of func
         """
-        parent_context = (
-            LoggingContext.current_context()
-        )  # type: Optional[LoggingContextOrSentinel]
-        if parent_context == LoggingContext.sentinel:
+        parent_context = current_context()  # type: Optional[LoggingContextOrSentinel]
+        if not parent_context:
             logger.warning(
                 "Starting db connection from sentinel context: metrics will be lost"
             )
@@ -1558,3 +1567,74 @@ def make_in_list_sql_clause(
         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