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b2c2b03079
PostgreSQL may underestimate the number of distinct `room_id`s in `event_search`, which can cause it to use table scans for queries for multiple rooms. Fix this by setting `n_distinct` on the column. Resolves #14402. Signed-off-by: Sean Quah <seanq@matrix.org>
34 lines
1.6 KiB
Plaintext
34 lines
1.6 KiB
Plaintext
/* Copyright 2022 The Matrix.org Foundation C.I.C
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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-- By default the postgres statistics collector massively underestimates the
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-- number of distinct rooms in `event_search`, which can cause postgres to use
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-- table scans for queries for multiple rooms.
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--
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-- To work around this we can manually tell postgres the number of distinct rooms
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-- by setting `n_distinct` (a negative value here is the number of distinct values
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-- divided by the number of rows, so -0.01 means on average there are 100 rows per
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-- distinct value). We don't need a particularly accurate number here, as a) we just
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-- want it to always use index scans and b) our estimate is going to be better than the
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-- one made by the statistics collector.
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ALTER TABLE event_search ALTER COLUMN room_id SET (n_distinct = -0.01);
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-- Ideally we'd do an `ANALYZE event_search (room_id)` here so that
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-- the above gets picked up immediately, but that can take a bit of time so we
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-- rely on the autovacuum eventually getting run and doing that in the
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-- background for us.
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