Re: Query takes way longer with LIMIT, and EXPLAIN takes way longer than actual query

On Fri, Aug 14, 2020 at 3:04 PM Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:

> Due to the over-estimated rowcount, the planner believes that (more) rows
> will
> be output (sooner) than they actually are:
>
> -> Nested Loop Semi Join (cost=47.11..31367302.81 ROWS=611586
> width=336) (actual time=47.098..97236.123 ROWS=25 loops=1)
>
> So it thinks there's something to be saved/gained by using a plan that has
> a
> low startup cost. But instead, it ends up running for a substantial
> fraction
> of the total (estimated) cost.
>
> Got it. Is there any way to address this other than re-writing the
query? (Statistics? Or something else?)

> As for the "explain is more expensive than the query", that could be due to
> timing overhead, as mentioned here. Test with "explain (timing off)" ?
> https://www.postgresql.org/docs/12/using-explain.html#USING-EXPLAIN-CAVEATS
>
>
Good call--explain with the timing off showed about the same time as the
actual query.

Thanks!

Ken

> --
> Justin
>

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