Misestimate when applying condition like id = id

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От Michael Lewis
Тема Misestimate when applying condition like id = id
Дата
Msg-id CAHOFxGouAogS2oRKscsqzd2XVH+10n3w3ihxRngjRd-wC9gvTA@mail.gmail.com
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Ответы Re: Misestimate when applying condition like id = id
Список pgsql-general
Simplified example query-
explain
select * from organizations o where id = id;

On a PG10 instance I get-
Seq Scan on organizations o (cost=0.00..4.21 ***rows=1*** width=37)
Filter: (id = id)

On a PG11 instance with the same or very very similar data, I get-
Seq Scan on organizations o  (cost=0.00..5.70 ***rows=270*** width=37)
  Filter: (id IS NOT NULL)

Given this is a not null field, it translates to "is true" basically. The real query that this example was extracted from apparently had a typo in the join resulting in this bogus id = id condition, but I still curious about where this estimate change comes from.


Michael Lewis  |  Database Engineer
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