| From: | Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk> |
|---|---|
| To: | MichaelDBA <MichaelDBA(at)sqlexec(dot)com> |
| Cc: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Justin Pryzby <pryzby(at)telsasoft(dot)com>, Ivan Voras <ivoras(at)gmail(dot)com>, postgres performance list <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: Optimising a two column OR check |
| Date: | 2019-10-12 16:58:22 |
| Message-ID: | 87imotx5uq.fsf@news-spur.riddles.org.uk |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
| Thread: | |
| Lists: | pgsql-performance |
>>>>> "MichaelDBA" == MichaelDBA <MichaelDBA(at)sqlexec(dot)com> writes:
MichaelDBA> Yep, you're right, Andrew, adding a couple rows made it do
MichaelDBA> the index only scan. I reckon I got misled by turning off
MichaelDBA> sequential scans, thinking that actual rows were not
MichaelDBA> important anymore. Overly simplistic reasonings can get
MichaelDBA> one into trouble, lol.
We do some odd stuff with the statistics estimates for completely empty
tables because (a) it's not common in practice for a table to be always
empty (i.e. the emptiness is usually transient) and (b) if you take the
emptiness of a table at face value, you end up generating insanely bad
plans for certain FK check queries that may not get replanned quickly
enough to mitigate the performance impact.
--
Andrew (irc:RhodiumToad)
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