Re: Example code Re: Singleton SELECT inside cursor loop

On 2022-10-01 02:22:22 -0500, Ron wrote:
> I extracted the singleton into a function, and that helped me isolate the
> problem.

I don't know what a singleton select is (duckduckgo tells me it has
something to do with DB2 and/or COBOL, or maybe it's a whisky), but:


> Note how quickly it runs the first five times, but takes 780x longer the
> sixth time I run it.  Exiting psql and entering again causes the same
> slowness the sixth time it's run.

PostgresSQL evaluates the exact plan the first 5 times it encounters the
same prepared query. After that it switches to a generic plan if it
thinks that the generic plan isn't worse than the specialized plans.

If the plan suddenly gets worse after 5 executions, you've probably run
into a case where the generic plan is worse although the cost computed
by the planner isn't.

        hp

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