On 15 January 2016 at 04:00, Tom Lane <tgl@xxxxxxxxxxxxx> wrote:
David Rowley <david.rowley@xxxxxxxxxxxxxxx> writes:
> Perhaps separating out enable_nestloop so that it only disables
> non-parameterised nested loops, and add another GUC for parameterised
> nested loops would be a good thing to do. Likely setting enable_nestloop to
> off in production would be a slightly easier decision to make, if that was
> the case.
> It looks pretty simple to do this, so I hacked it up, and attached it here.
> There's no doc changes and I'm not that interested in fighting for this
> change, it's more just an idea for consideration.
I'm not terribly excited by this idea either. If making such a change
actually makes things better for someone consistently, I'd argue that
the problem is a mistaken cost estimate elsewhere, and we'd be better off
to find and fix the real problem. (There have already been discussions
of only believing single-row rowcount estimates when they're provably
true, which might help if we can figure out how to do it cheaply enough.)
Actually, it's not very hard to hit a bad underestimate at all. All you need is a join on two columns which are co-related. Since PostgreSQL multiplies the estimated selectivities the row count is going to come out too low. This also tricks the planner into thinking that this is a good join to perform early, since (it thinks that) it does not produce many rows at all. You only need 1 more join to occur after that to choose a nested loop join mistakenly to hit the issue.
FWIW TPC-H Q9 has this exact trip hazard with the partsupp table, which is the exact reason why this patch was born: https://commitfest.postgresql.org/7/210/
I also think that the attitude that we can *always* fix the costs and estimates is not the right one. The planner is never going to get it right 100% of the time. If we ever think we can build such a planner then someone needs to come along and direct us back into the real world.
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