Re: Slow planning, fast execution for particular 3-table query

On Mon, 2019-11-04 at 03:04 +0000, David Wheeler wrote:
> We’re having trouble working out why the planning time for this particular query is slow
> (~2.5s vs 0.9ms execution time). As you can see below, there are only 3 tables involved
> so it’s hard to imagine what decisions the planner has to make that take so long. After
> 5 runs the prepared-statement code kicks in and it becomes quick, but it’s quite
> infuriating for the first 5 runs given the execution is so much faster.
>
> Are you able to give any tips what might be taking so long (and how we might improve it)?
>
[...]
> Planning time: 2429.682 ms
>
> Execution time: 0.914 ms

Strange.
Are any of your catalog tables unusually large?

SELECT pg_relation_size(t.oid),
t.relname
FROM pg_class AS t
JOIN pg_namespace AS n ON t.relnamespace = n.oid
WHERE t.relkind = 'r'
ORDER BY pg_relation_size(t.oid) DESC
LIMIT 10;

Yours,
Laurenz Albe
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