Unverified Commit 3acf85c8 authored by Erik Johnston's avatar Erik Johnston Committed by GitHub
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Reduce likelihood of Postgres table scanning `state_groups_state`. (#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.
parent 9f497024
Fix PostgreSQL sometimes using table scans for queries against `state_groups_state` table, taking a long time and a large amount of IO.
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-- 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.
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