On Wed, Dec 3, 2014 at 11:03 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Stephen Frost <sfrost@snowman.net> writes: > * Tom Lane (tgl@sss.pgh.pa.us) wrote: >> However, even granting that that is a concern, so what? You *have* to >> do the planning twice, or you're going to be generating a crap plan for >> one case or the other.
> Yeah, I don't see a way around that..
Also, it occurs to me that it's only necessary to repeat the join search part of the process, which means that in principle the mechanisms already exist for that; see GEQO. This means that for small join problems, the total planning time would much less than double anyway. For large problems, where the join search is the bulk of the time, we could hope that removal of unnecessary joins would reduce the join search runtime enough that the second search would be pretty negligible next to the first (which is not optional). So I think "it'll double the runtime" is an unfounded objection, or at least there's good reason to hope it's unfounded.
Is it possible to only replan part of the plan in case of this optimization? I think that we might need to only replan parts of the original plan (as you mentioned, join search and above). So we could reuse the original plan in part and not do a lot of replanning (an obvious case is scan strategy, which we can assume will not change for the two plans).
I wonder if we could have a rule based system for replacement of some plan nodes with other type of nodes. As we discover more cases like this, we could add more rules. Wild thought though.
