Re: Better performance possible for a pathological query?

=?UTF-8?B?QWxleGlzIEzDqi1RdcO0Yw==?= <alq(at)datadoghq(dot)com> writes:
> The query itself is very simple: a primary key lookup on a 1.5x10^7 rows.
> The issue is that we are looking up over 11,000 primary keys at once,
> causing the db to consume a lot of CPU.

It looks like most of the runtime is probably going into checking the
c.key = ANY (ARRAY[...]) construct. PG isn't especially smart about that
if it fails to optimize the construct into an index operation --- I think
it's just searching the array linearly for each row meeting the other
restrictions on c.

You could try writing the test like this:
c.key = ANY (VALUES (1), (17), (42), ...)
to see if the sub-select code path gives better results than the array
code path. In a quick check it looked like this might produce a hash
join, which seemed promising anyway.

regards, tom lane