Re: [Bug][Ver 11]: Generic query plan selected is worse than custom query plan

Malay Keshav <malay(dot)keshav(at)databricks(dot)com> writes:
> In our particular case, the Postgres engine decided to pick the generic
> query plan and cache it for all further calls with that query. My
> understanding was that the generic query plan would only be selected if it
> had a better execution time than the custom query plan. Which in our case
> is not true.

> We were able to reproduce this deterministically using the same query
> parameters to trigger the engine to pick the bad generic query plan on the
> 6th run (first 5 runs shows the engine used the efficient query plan). Why
> does the engine pick the generic query plan when its execution time is
> worse than the custom query plan? Is this a bug?

Since you've provided precisely zero detail, it's impossible to answer
in any detail, but this is unlikely to be anything we'd call a bug.
What's presumably happening is that the planner's estimate of the cost
of the generic plan comes out less than its estimate of the cost of
a custom plan, but that estimate is faulty. Sure, we'd love to improve
all such situations, but it's not an easy task --- and any changes in that
area would not be something we'd consider back-patching into long-stable
branches.

regards, tom lane