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Override global statement timeout when creating indexes in Postgres #16085

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Aug 17, 2023
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Override global statement timeout when creating indexes in Postgres #16085

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5d51db6
disable global statement timeout when creating index
H-Shay Aug 8, 2023
81a36d7
newsfragement
H-Shay Aug 8, 2023
6de8215
fix syntax
H-Shay Aug 8, 2023
c18bc5c
use `session` rather than `local` for statement_timeout
H-Shay Aug 15, 2023
64a4228
move return to default to finally block
H-Shay Aug 16, 2023
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1 change: 1 addition & 0 deletions changelog.d/16085.misc
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Override global statement timeout when creating indexes in Postgres.
11 changes: 11 additions & 0 deletions synapse/storage/background_updates.py
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Expand Up @@ -238,6 +238,7 @@ class BackgroundUpdater:
def __init__(self, hs: "HomeServer", database: "DatabasePool"):
self._clock = hs.get_clock()
self.db_pool = database
self.hs = hs

self._database_name = database.name()

Expand Down Expand Up @@ -758,6 +759,11 @@ def create_index_psql(conn: Connection) -> None:
logger.debug("[SQL] %s", sql)
c.execute(sql)

# override the global statement timeout to avoid accidentally squashing
# a long-running index creation process
timeout_sql = "SET SESSION statement_timeout = 0"
c.execute(timeout_sql)
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We need to set this back to the default, as we're not in a transaction when creating indices I think?

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Is the autocommit being on the reason that this isn't in a transaction? The statement timeout only applies for a given transaction - if we are not in a transaction then I wonder if it will have any effect. I will do some digging in the docs but they were pretty scarce on information when I looked into this the first time. Do we have any Postgres/psychopg wizards around who might have some arcane insight into this?

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Is the autocommit being on the reason that this isn't in a transaction?

Yes

The statement timeout only applies for a given transaction - if we are not in a transaction then I wonder if it will have any effect.

https://www.postgresql.org/docs/current/sql-set.html says that LOCAL doesn't work outside a transaction, and so we'd need to use SESSION


sql = (
"CREATE %(unique)s INDEX CONCURRENTLY %(name)s"
" ON %(table)s"
Expand All @@ -772,6 +778,11 @@ def create_index_psql(conn: Connection) -> None:
logger.debug("[SQL] %s", sql)
c.execute(sql)

# mypy ignore - `statement_timeout` is defined on PostgresEngine
default_timeout = self.db_pool.engine.statement_timeout # type: ignore[attr-defined]
undo_timeout_sql = f"SET statement_timeout = {default_timeout}"
c.execute(undo_timeout_sql)
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This needs to be in finally section, so if happens even if index creation raises an exception


if replaces_index is not None:
# We drop the old index as the new index has now been created.
sql = f"DROP INDEX IF EXISTS {replaces_index}"
Expand Down