Re: Immutable function WAY slower than Stable function?

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От	Ken Tanzer
Тема	Re: Immutable function WAY slower than Stable function?
Дата	7 августа 2018 г. 13:05:39
Msg-id	CAD3a31Vwg5yDzTi7ALhZ9Y2WMHTYC-1VXTPGdNcUR+XW5pyC_Q@mail.gmail.com обсуждение исходный текст
Ответ на	Re: Immutable function WAY slower than Stable function? (Laurenz Albe <laurenz.albe@cybertec.at>)
Ответы	Re: Immutable function WAY slower than Stable function? (Ken Tanzer <ken.tanzer@gmail.com>) Re: Immutable function WAY slower than Stable function? (Laurenz Albe <laurenz.albe@cybertec.at>)
Список	pgsql-general

Дерево обсуждения

On Mon, Aug 6, 2018 at 11:05 PM Laurenz Albe <laurenz.albe@cybertec.at> wrote:

Ken Tanzer wrote:
> On Mon, Aug 6, 2018 at 4:11 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > Ken Tanzer <ken.tanzer@gmail.com> writes:
>
> spc=> EXPLAIN (ANALYZE,BUFFERS,VERBOSE) SELECT client_id,si_imm(client_id) FROM tbl_residence_own;
> QUERY PLAN
> -------------------------------------------------------------------------------------------------------------------------------
> Seq Scan on public.tbl_residence_own (cost=0.00..1990.02 rows=6977 width=8) (actual time=3.771..22665.604 rows=6983 loops=1)
> Output: client_id, si_imm(client_id)
> Buffers: shared hit=199814
> Planning time: 0.156 ms
> Execution time: 22677.333 ms
> (5 rows)
>
> spc=> EXPLAIN (ANALYZE,BUFFERS,VERBOSE) SELECT client_id,si_stable(client_id) FROM tbl_residence_own;
> QUERY PLAN
> ------------------------------------------------------------------------------------------------------------------------------
> Seq Scan on public.tbl_residence_own (cost=0.00..3734.27 rows=6977 width=8) (actual time=3.100..1302.888 rows=6983 loops=1)
> Output: client_id, staff_inspector_stable(client_id, target_date())
> Buffers: shared hit=60174
> Planning time: 0.354 ms
> Execution time: 1315.746 ms
> (5 rows)

That seems to say that the _stable function is running much faster.

Agreed!

Buth functions don't get inlined.

OK, I clearly don't understand the output lines. What does it mean then that the stable output line shows the underlying (two-argument) function, while the immutable one shows the convenience function?

Output: client_id, si_imm(client_id)

Output: client_id, staff_inspector_stable(client_id, target_date())

I'd dig into the functions and find out how long the queries in
them take. auto_explain is a good helper for that.

I'm definitely not understanding why or how auto-explain would help here. (Also, don't overlook the fact that both si_stable and si_imm have the exact same definition (except for stable v. immutable), and are calling the same function (staff_inspector_stable)). I can load up the auto_explain stuff if it really will help, but in the meantime what I did was pull the query out of staff_inspector_stable, and inserted into the "SELECT client_id,staff_inspector( ... ) FROM tbl_residence_own" query, replacing the "client" (parameter 1) with the client_id from the tbl_residence_own, and replacing "asof" (parameter 2) with target_date(), which seems like it should closely mimic the original query.

I ended up with the query below, and have attached the explain output to this email. The actual timing is:

 Planning time: 2.737 msExecution time: 821.034 ms

Which is somewhat better than the function versions, though it seems unsurprisingly and not dramatically so.

EXPLAIN (VERBOSE,ANALYZE,BUFFERS) SELECT client_id,
CASE WHEN
(SELECT program_type_code FROM reg_spc WHERE target_date() BETWEEN reg_spc_date AND COALESCE(reg_spc_date_end,target_date()) AND client_id=tro.client_id LIMIT 1)
ILIKE 'SSP%' THEN   COALESCE((SELECT staff_id FROM staff_employment_current WHERE staff_position_code='COORD_PROP' AND agency_project_code='SSP' LIMIT 1),(SELECT staff_id FROM staff_employment_current WHERE staff_position_code='MGRPROJ' AND agency_project_code='SSP' LIMIT 1))
ELSE   (SELECT staff_inspector FROM tbl_residence_own ro LEFT JOIN l_housing_project USING (housing_project_code) WHERE client_id=5538 AND target_date() BETWEEN residence_date AND COALESCE(residence_date_end,target_date()) AND NOT ro.is_deleted LIMIT 1)
END
FROM tbl_residence_own tro;

Let me know if that is not helpful. Or if you need something from auto-explain, please help me with some more specifics. Thanks!

Ken