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List:       pgsql-performance
Subject:    Re: Strange behavior of limit clause in complex query
From:       Justin Pryzby <pryzby () telsasoft ! com>
Date:       2022-06-08 14:32:47
Message-ID: 20220608143247.GO29853 () telsasoft ! com
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On Wed, Jun 08, 2022 at 09:44:08AM +0100, Paulo Silva wrote:
> But if I add an ORDER BY and a LIMIT something goes very wrong (Q2):

A somewhat common problem.

A common workaround is to change "ORDER BY a" to something like "ORDER BY a+0"
(if your framework will allow it).

> An EXPLAIN (ANALYZE, BUFFERS) for Q2 returns this:
...
> ->  Index Scan Backward using ix_ng_content_date on ng_content "Extent1"  \
> (cost=0.43..40616715.85 rows=2231839 width=12) (actual time=11027.808..183839.289 \
>                 rows=5 loops=1)
> Filter: ((2 = id_status) AND (date_from <= LOCALTIMESTAMP) AND (date_to >= \
> LOCALTIMESTAMP) AND (SubPlan 1)) Rows Removed by Filter: 4685618
> Buffers: shared hit=15414533 read=564480 written=504

I'm not sure if it would help your original issue, but the rowcount estimate
here is bad - overestimating 2231839 rows instead of 5.

Could you try to determine which of those conditions (id_status, date_from,
date_to, or SubPlan) causes the mis-estimate, or if the estimate is only wrong
when they're combined ?

-- 
Justin


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