On Mon, Jun 26, 2023 at 4:00 PM Andres Freund <andres@anarazel.de> wrote:
>
> Hi,
>
> On 2023-06-26 13:40:49 -0400, James Coleman wrote:
> > Have we ever discussed running an analyze immediately after creating a table?
>
> That doesn't make a whole lot of sense to me - we could just insert the
> constants stats we wanted in that case.
>
I thought that was implicit in that, but fair enough :)
> > Consider the following:
> >
> > create table stats(i int, t text not null);
> > explain select * from stats;
> > Seq Scan on stats (cost=0.00..22.70 rows=1270 width=36
> > analyze stats;
> > explain select * from stats;
> > Seq Scan on stats (cost=0.00..0.00 rows=1 width=36)
> >
> > Combined with rapidly increasing error margin on row estimates when
> > adding joins means that a query joining to a bunch of empty tables
> > when a database first starts up can result in some pretty wild plan
> > costs.
>
> The issue is that the table stats are likely going to quickly out of date in
> that case, even a hand full of inserts (which wouldn't trigger
> autovacuum analyzing) would lead to the "0 rows" stats causing very bad plans.
>
It's not obvious to me (as noted elsewhere in the thread) which is
worse: a bunch of JOINs on empty tables can result in (specific
example) plans with cost=15353020, and then trigger JIT, and...here we
collide with my other thread about JIT [1].
Regards,
James Coleman
1: https://www.postgresql.org/message-id/CAAaqYe-g-Q0Mm5H9QLcu8cHeMwok%2BHaxS4-UC9Oj3bK3a5jPvg%40mail.gmail.com