Re: Understanding query planner cpu usage

On Wed, Feb 21, 2018 at 7:28 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> > What is the planner doing when trying to find the current live max value
> of
> > the column?
>
> It's trying to estimate whether a mergejoin will be able to stop short of
> reading all the tuples from the other side of the join. (For instance,
> if you've got 1,3,5 on one side, and 1,4,5,7,8,9,19 on the other, the
> second input doesn't have to be read past "7" because once we run off the
> end of the first input, we know we couldn't see any matches later on the
> second input. So the planner wants to compare the ending key value on
> each side to the key distribution on the other side, to see what this might
> save.) Now, that's a unidirectional question for any particular mergejoin
> plan, so that for any one cost estimate it's only going to need to look at
> one end of the key range. But I think it will consider merge joins with
> both sort directions, so that both ends of the key range will get
> investigated in this way. I might be wrong though; it's late and I've
> not looked at that code in awhile ...
>

I'm thinking the least painful solution here might be to set
enable_mergejoin = false for this particular query, since the rows joined
are quite sparse.