Re: LIMIT causes SEQSCAN in subselect

From: Pierre-Frédéric Caillaud
Subject: Re: LIMIT causes SEQSCAN in subselect
Date: ,
Msg-id: opsjholqqlcq72hf@musicbox
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In response to: Re: LIMIT causes SEQSCAN in subselect  (Josh Berkus)
List: pgsql-performance

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LIMIT causes SEQSCAN in subselect  (Mike Rylander, )
 Re: LIMIT causes SEQSCAN in subselect  (Josh Berkus, )
  Re: LIMIT causes SEQSCAN in subselect  (Mike Rylander, )
  Re: LIMIT causes SEQSCAN in subselect  (Tom Lane, )
  Re: LIMIT causes SEQSCAN in subselect  (Pierre-Frédéric Caillaud<>, )

> The fact that the estimator knows that the LIMIT is pointless because
> there
> are less rows in the subselect than the LIMIT will return is not
> something we
> want to count on; sometimes the estimator has innaccurate information.
> The
> UNIQUE index makes this more certain, except that I'm not sure that the
> planner distinguishes between actual UNIQUE indexes and columns which are
> estimated unique (per the pg_stats).   And I think you can see in your
> case
> that there's quite a difference between a column we're CERTAIN is unique,
> versus a column we THINK is unique.

    I think a UNIQUE constraint can permit several 'different' NULL values...
better say "UNIQUE NOT NULL" ?



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