On Wed, Feb 21, 2018 at 7:28 PM, Tom Lane <t...@sss.pgh.pa.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.
