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

Awesome, thanks! I will give that a shot

On Wed, 24 May 2017 at 6:14 pm, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Jeff Janes <jeff(dot)janes(at)gmail(dot)com> writes:
> > On Wed, May 24, 2017 at 1:42 PM, Sam Saffron <sam(dot)saffron(at)gmail(dot)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
>