Re: Strange query planner behavior

Re

Indeed the distribution of data is very concentrated with value ID_MASTER =
0 for 99% of the table
I put the example of (SELECT 10) after some testing to show the strangeness
of the situation but originally it was a subrequest on other tables. I
understand the planner isn't conceived to expect a "select CONST" but a
subrequest giving only one result (you know it will be only 1 result
because there is a = operator before, if it could produce several results
the request would fail) stays constant along the query so it'd be better in
some cases if calculated before setting up the query plan of the rest
I rewrote my stored procedure to get the result of the subrequest then
select * from livraison where id_master = the result and everything works
fine using the index. Thanks

Maybe an idea for future optimisation in the query planner would be that
every subrequest that has a = before or any other operator allowing only
0-1 result (<, >, IS NULL, etc.) could be calculated before setting up the
query plan of the higher level (and could be in parallel if the subrequest
is big work). Then the subrequest result would not be 'unknown' anymore and
used for query plan setup

For example:

SELECT * FROM TABLE_A WHERE CONDITIONS AND COLUMN1 = (SELECT WHATEVER FROM
TABLE_B INNER JOIN TABLE_C ON (...) WHERE ... etc) would set up a query
plan like:

1) Calculate a plan and estimate the complexity of the subrequest(s). Even
better: if very complex launch it(them) in another thread(s) / process(es)
2) Estimate if CONDITIONS will remove a lot of rows in TABLE_A. If yes
(case A) subrequest result will be used only for recheck and we can set up
and launch the main query plan at once. If not (case B) we have to wait for
the subrequest to be finished
3) If case A and no rows to be rechecked (so no result) then finished. Else
wait for the subrequest result (if parallel) or calculate it (if not
parallel) as it will be needed in step 4
4) In case A recheck according to the subrequest result and in case B set
up a query plan for the main request that takes into accounts the result of
the subrequest and pray that it gives far better performance. Or maybe
limit that to simple cases where the column compared to the subrequest is
indexed and it will be undoubtfully be faster

I'm not expert enough to imagine all cases especially if stored procedures
that modify data are implied, maybe it's not feasible. But it's an idea

Have a nice day and thanks
Martin

Le sam. 30 nov. 2019 à 11:46, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> a
écrit :

>
>
> so 30. 11. 2019 v 11:29 odesílatel EffiSYS / Martin Querleu <
> martin(dot)querleu(at)effisys(dot)fr> napsal:
>
>> Hi Pavel
>>
>> Thanks for the fast reply
>> Our databases are VACUUMed everyday. I did it again but no difference
>>
>> Here are the query plans:
>>
>> EFT_MBON=# explain analyse select * from livraison where id_master = 10;
>> QUERY
>> PLAN
>>
>> --------------------------------------------------------------------------------------------------------------------------------------------
>> Index Scan using pour_recherche_sous_livraison on livraison
>> (cost=0.03..15.04 rows=1 width=697) (actual time=0.017..0.017 rows=0
>> loops=1)
>> Index Cond: (id_master = 10)
>> Planning Time: 0.124 ms
>> Execution Time: 0.036 ms
>> (4 lignes)
>>
>> EFT_MBON=# explain analyse select * from livraison where id_master =
>> (select 10);
>> QUERY PLAN
>>
>>
>> -----------------------------------------------------------------------------------------------------------------------
>> Seq Scan on livraison (cost=0.01..2888156.69 rows=1917632 width=697)
>> (actual time=1334.615..1334.615 rows=0 loops=1)
>> Filter: (id_master = $0)
>> Rows Removed by Filter: 1918196
>> InitPlan 1 (returns $0)
>> -> Result (cost=0.00..0.01 rows=1 width=4) (actual
>> time=0.000..0.001 rows=1 loops=1)
>> Planning Time: 0.138 ms
>> Execution Time: 1334.642 ms
>> (7 lignes)
>>
>> Regarding the cost calculator the configuration is as follows:
>>
>> random_page_cost and seq_page_cost are identical since the data is 100%
>> in RAM (both at 15.0, 3 times default)
>> cpu_tuple_cost at 0.005 (half default)
>> cpu_index_tuple_cost at 0.00025 (half defaut)
>> cpu_operator_cost at 0.00025 (default, by the way I assume we should
>> lower it at 0.0001)
>>
>> I would expect the seq scan to be more costly than default since both
>> page_cost are higher and cpu_index_tuple_cost lower
>>
>> I think the main question is whether the query planner is able to pre
>> calculate subqueries with = to use the value returned to get the good query
>> plan
>>
>
> The basic problem is in very bad estimation
>
> Seq Scan on livraison (cost=0.01..2888156.69 rows=1917632 width=697)
> (actual time=1334.615..1334.615 rows=0 loops=1)
>
> Looks like the estimation lost a const value, and try to estimate result
> against unknown variable. Probably the table livraison has in id_master
> some values that has massively higher number than other. Subplans are
> estimated separately.
>
> There is not simply solution - you have to rewrite your queries - used
> syntax blocks flattening, and that is wrong.
>
> SELECT * FROM LIVRAISON WHERE ID_MASTER = (SELECT 10)
>
> this query is optimized as two independent queries - SELECT * FROM
> LIVRAISON WHERE ID_MASTER and SELECT 10. Although "SELECT 10" has const
> result, first query desn't calculate it. Postgres planner doesn't expect so
> somebody will write these queries, and don't try to detect const table
> results. If you rewrite query to
>
> SELECT * FROM LIVRAISON WHERE ID_MASTER IN (SELECT 10)
>
> Then it will be optimized as one query and it should to work.
>
> Pavel
>
>
>
>
>
>
>
>
>
>>
>> Best regards
>> Martin
>>
>> On 30/11/2019 11:00, Pavel Stehule wrote:
>>
>>
>>
>> so 30. 11. 2019 v 10:55 odesílatel Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
>> napsal:
>>
>>> Hi
>>>
>>> so 30. 11. 2019 v 10:31 odesílatel EffiSYS / Martin Querleu <
>>> martin(dot)querleu(at)effisys(dot)fr> napsal:
>>>
>>>> Hello
>>>>
>>>> I have a strange problem with the query planner on Postgresql 11.5 on
>>>> Debian stretch, the plan differs between the following 2 requests:
>>>>
>>>> - SELECT * FROM LIVRAISON WHERE ID_MASTER = 10 which uses a btree index
>>>> on ID_MASTER (the table has 1M rows). Everything is normal
>>>> - SELECT * FROM LIVRAISON WHERE ID_MASTER = (SELECT 10) which uses a
>>>> seq
>>>> scan and is 3000 times slower
>>>>
>>>> I don't understand how the planner cannot consider that a subselect
>>>> with
>>>> an = is equivalent to having = VALUE (the subselect either returning 1
>>>> row or NULL)
>>>>
>>>> I don't have the same behavior on other column with indexes of the same
>>>> table, maybe it's because 99% or the table has ID_MASTER = 0? I can
>>>> understand that if the value returned by the subquery is 0 the seqscan
>>>> could be faster (in our case it is still slower than index scan but
>>>> only
>>>> by 2 times), but if the subquery does not return 0 in no case the
>>>> seqscan could be faster. The question is why is the subquery not
>>>> calculated before choosing wether to use the index or not since it will
>>>> return a single value?
>>>>
>>>> Thanks for your reply and sorry if the question is stupid
>>>>
>>>
>>> please try
>>>
>>> 1. run vacuum analyze on LIVRAISON
>>> 2. send result of EXPLAIN ANALYZE SELECT * FROM ... for both cases
>>>
>>
>> 3. do you have some custom settings of planner configuration variables
>> like random_page_cost, seq_page_cost?
>>
>>
>>> here is a tool for sharing explains https://explain.depesz.com/
>>>
>>> Regards
>>>
>>> Pavel
>>>
>>>>
>>>> Best regards
>>>> Martin Querleu
>>>>
>>>>
>>>>
>>
>> --
>> Martin Querleu - Directeur Général
>> EffiSYS (www.effitrace.fr - www.logistique-e-commerce.fr)martin(dot)querleu(at)effisys(dot)fr
>> 3, rue Gustave Delory
>> 59000 Lille
>> Tél: +33 9 54 28 38 76
>>
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>>