Re: view reading information_schema is slow in PostgreSQL 12

On Fri, Jun 12, 2020 at 11:11:09PM -0400, Tom Lane wrote:
> regrog <andrea(dot)vencato(at)gmail(dot)com> writes:
> > I'm facing performance issues migrating from postgres 10 to 12 (also from 11
> > to 12) even with a new DB.
> > The simple query: select * from my_constraints is normal but as soon as I
> > add where constraint_type = 'FOREIGN KEY' it takes a lot of time.
>
> I looked at this a bit. I see what's going on, but I don't see an easy
> workaround :-(. The information_schema.table_constraints view contains
> a UNION ALL, which in your v10 query produces this part of the plan:

> To get a decent plan out of v12, the problem is to get it to produce
> a better rowcount estimate for the first arm of table_constraints'
> UNION. We don't necessarily need it to match the 1800 reality, but
> we need it to be more than 1. Unfortunately there's no simple way
> to affect that. The core misestimate is here:

> I expect you're getting a fairly decent estimate for the "contype <>
> ALL" condition, but the planner has no idea what to make of the CASE
> construct, so it just falls back to a hard-wired default estimate.
>
> I don't have any good suggestions at the moment. If you had a lot more
> tables (hence more rows in pg_constraint) the plan would likely shift
> to something tolerable even with the crummy selectivity estimate for the
> CASE. But where you are, it's hard. A conceivable workaround is to
> drop the "tc.constraint_type <> 'CHECK'" condition from your view, which
> would resurrect that UNION arm and probably get you back to something
> similar to the v10 plan.

For the purposes of making this work for v12, you might try to look at either a
temporary table:

CREATE TEMP TABLE constraints AS SELECT * FROM information_schema.table_constraints WHERE constraint_type='FOREIGN KEY';
ANALYZE constraints;
SELECT * FROM ... LEFT JOIN constraints LEFT JOIN ...

or a CTE (which, if it works, is mostly dumb luck):
WITH constraints AS MATERIALIZED (SELECT * FROM information_schema.table_constraints) SELECT * FROM constraints WHERE constraint_type='FOREIGN KEY';

Or make a copy of the system view with hacks for the worst misestimates (like
contype<>'c' instead of constraint_type<>'CHECK').

--
Justin