35 lines
1.6 KiB
Plaintext
35 lines
1.6 KiB
Plaintext
/* Copyright 2021 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 state groups are in the `state_groups_state`, which can
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-- cause postgres to use table scans for queries for multiple state groups.
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--
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-- To work around this we can manually tell postgres the number of distinct state
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-- groups there are by setting `n_distinct` (a negative value here is the number
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-- of distinct values divided by the number of rows, so -0.02 means on average
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-- there are 50 rows per distinct value). We don't need a particularly
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-- accurate number here, as a) we just want it to always use index scans and b)
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-- our estimate is going to be better than the one made by the statistics
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-- collector.
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ALTER TABLE state_groups_state ALTER COLUMN state_group SET (n_distinct = -0.02);
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-- Ideally we'd do an `ANALYZE state_groups_state (state_group)` 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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