Skip to content
This repository has been archived by the owner on Apr 26, 2024. It is now read-only.

Commit

Permalink
Reduce likelihood of Postgres table scanning state_groups_state. (#…
Browse files Browse the repository at this point in the history
…10359)

The postgres statistics collector sometimes massively underestimates the
number of distinct state groups are in the `state_groups_state`, which
can cause postgres to use table scans for queries for multiple state
groups.

We fix this by manually setting `n_distinct` on the column.
  • Loading branch information
erikjohnston authored Jul 15, 2021
1 parent 9f49702 commit 3acf85c
Show file tree
Hide file tree
Showing 2 changed files with 35 additions and 0 deletions.
1 change: 1 addition & 0 deletions changelog.d/10359.bugfix
Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Fix PostgreSQL sometimes using table scans for queries against `state_groups_state` table, taking a long time and a large amount of IO.
Original file line number Diff line number Diff line change
@@ -0,0 +1,34 @@
/* Copyright 2021 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.
*/


-- By default the postgres statistics collector massively underestimates the
-- number of distinct state groups are in the `state_groups_state`, which can
-- cause postgres to use table scans for queries for multiple state groups.
--
-- To work around this we can manually tell postgres the number of distinct state
-- groups there are by setting `n_distinct` (a negative value here is the number
-- of distinct values divided by the number of rows, so -0.02 means on average
-- there are 50 rows per distinct value). We don't need a particularly
-- accurate number here, as a) we just want it to always use index scans and b)
-- our estimate is going to be better than the one made by the statistics
-- collector.

ALTER TABLE state_groups_state ALTER COLUMN state_group SET (n_distinct = -0.02);

-- Ideally we'd do an `ANALYZE state_groups_state (state_group)` here so that
-- the above gets picked up immediately, but that can take a bit of time so we
-- rely on the autovacuum eventually getting run and doing that in the
-- background for us.

0 comments on commit 3acf85c

Please sign in to comment.