Re: [GENERAL] Why is posgres picking a suboptimal plan for this query?

Jeff Janes <jeff.janes@gmail.com> writes:
> On Wed, May 24, 2017 at 1:42 PM, Sam Saffron <sam.saffron@gmail.com> wrote:
>> I have this query that is not picking the right index unless I hard code
>> dates:
>> ...

> Maybe it should first execute the subquery and then re-plan the rest of the
> query based on the results.  But there is no provision for it to do that,
> and no concrete plans (that I know of) to implement such a thing.

I don't know of any plans for that, either.

>> The results here simply do not make sense to me, should I be piping
>> dates in here to avoid this issue and running 2 queries instead of 1?

> That is the most pragmatic approach.  It isn't very nice, but the
> alternatives are worse.

You could probably get the behavior you want by replacing the subquery
with a "stable" function:

create function first_topic_unread_for(userid int) returns timestamp as
'select first_topic_unread_at from user_stats us where us.user_id = $1'
language sql stable;

SELECT "topics".* FROM "topics"
WHERE topics.last_unread_at >= first_topic_unread_for(1);

This should convince the planner to pre-run the function to get an
estimated result at plan time.

            regards, tom lane