Re: Hash Join over Nested Loop

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pá 22. 11. 2019 v 19:42 odesílatel Luís Roberto Weck <luisroberto@xxxxxxxxxxxxxxx> napsal:
Em 22/11/2019 14:55, Pavel Stehule escreveu:


pá 22. 11. 2019 v 18:37 odesílatel Luís Roberto Weck <luisroberto@xxxxxxxxxxxxxxx> napsal:
Hey,

I'm trying to figure out why Postgres is choosing a Hash Join over a Nested Loop in this query:

SELECT T1.PesID, T1.PesNom, T1.PesValSal, T1.PesCPFCNPJ, T2.CarAti, T1.CarCod, T1.EmpCod,
       T2.CarFan, T1.PesDatAge, T1.PesCod,
COALESCE( T3.PesDatAnt, DATE '00010101') AS PesDatAnt
  FROM ((public.Pessoa T1
            INNER JOIN public.Carteira T2 ON T2.EmpCod = T1.EmpCod AND T2.CarCod = T1.CarCod)
             LEFT JOIN  (SELECT MIN(COALESCE( T5.ConVenAnt, DATE '00010101')) AS PesDatAnt, T4.EmpCod, T4.CarCod, T4.ConPesCod AS ConPesCod
                           FROM (public.Contrato T4
                                 LEFT JOIN  (SELECT MIN(ConParDatVen) AS ConVenAnt, EmpCod, CarCod, ConPesCod, ConSeq
                                               FROM public.ContratoParcela T5
                                              WHERE ConParAti = true
                                                AND ConParValSal > 0
                                              GROUP BY EmpCod, CarCod, ConPesCod, ConSeq ) T5 ON T5.EmpCod    = T4.EmpCod    AND
                                                                                                 T5.CarCod    = T4.CarCod    AND
                                                                                                 T5.ConPesCod = T4.ConPesCod AND
                                                                                                 T5.ConSeq    = T4.ConSeq)

                          WHERE T4.ConAti = TRUE
                          GROUP BY T4.EmpCod, T4.CarCod, T4.ConPesCod ) T3 ON t3.EmpCod    = T1.EmpCod AND
                                                                              t3.CarCod    = T1.CarCod AND
                                                                              t3.ConPesCod = T1.PesCod)

 WHERE (T2.CarAti = true)
   AND (T1.EmpCod = 112)
   and (UPPER(T1.PesNom) like UPPER('%MARIA%'))
 ORDER BY T1.EmpCod, T1.CarCod, T1.PesCod

Here the Hash Join[1] plan takes ~700ms, and if I change the first LEFT JOIN to a LEFT JOIN LATERAL, forcing a nested loop, the query[2] runs in 3ms.

[1]
https://explain.depesz.com/s/8IL3
[2]
https://explain.depesz.com/s/f8Q9

Maybe I am wrong, but probably you have to do more than just change LEFT JOIN to LATERAL JOIN. Lateral join is based on correlated subquery - so you had to push some predicates to subquery - and then the query can be much more effective.

Regards

Pavel





PostgreSQL version is 11.5, I have run analyze on all the tables.

PG settings:

name                           |setting  |unit|
-------------------------------|---------|----|
autovacuum                     |on       |    |
default_statistics_target      |250      |    |
effective_cache_size           |983040   |8kB |
effective_io_concurrency       |200      |    |
max_parallel_workers           |6        |    |
max_parallel_workers_per_gather|3        |    |
random_page_cost               |1.1      |    |
work_mem                       |51200    |kB  |

I'm sorry, I am not sure I understood.

This is the altered query:

SELECT T1.PesID, T1.PesNom, T1.PesValSal, T1.PesCPFCNPJ, T2.CarAti, T1.CarCod, T1.EmpCod, T2.CarFan, T1.PesDatAge, T1.PesCod,
       COALESCE( T3.PesDatAnt, DATE '00010101') AS PesDatAnt
  FROM ((public.Pessoa T1
            INNER JOIN public.Carteira T2 ON T2.EmpCod = T1.EmpCod AND T2.CarCod = T1.CarCod)
             LEFT JOIN LATERAL (SELECT MIN(COALESCE( T5.ConVenAnt, DATE '00010101')) AS PesDatAnt, T4.EmpCod, T4.CarCod, T4.ConPesCod AS ConPesCod
                                  FROM (public.Contrato T4
                                        LEFT JOIN  (SELECT MIN(ConParDatVen) AS ConVenAnt, EmpCod, CarCod, ConPesCod, ConSeq
                                                     FROM public.ContratoParcela T5
                                                    WHERE ConParAti = true
                                                      and ConParValSal > 0
                                                     
                                                    GROUP BY EmpCod, CarCod, ConPesCod, ConSeq ) T5 ON  T5.EmpCod = T4.EmpCod AND T5.CarCod = T4.CarCod AND T5.ConPesCod = T4.ConPesCod AND T5.ConSeq = T4.ConSeq)
                                 WHERE T4.ConAti = TRUE
                                   AND t4.EmpCod = T1.EmpCod AND t4.CarCod = T1.CarCod AND t4.ConPesCod = T1.PesCod
                                 GROUP BY T4.EmpCod, T4.CarCod, T4.ConPesCod ) T3 ON TRUE ) --ON t3.EmpCod = T1.EmpCod AND t3.CarCod = T1.CarCod AND t3.ConPesCod = T1.PesCod)
 WHERE (T2.CarAti = true)
   AND (T1.EmpCod = 112)
   and (UPPER(T1.PesNom) like UPPER('%MARIA%'))
 ORDER BY T1.EmpCod, T1.CarCod, T1.PesCod

In bold are the changes I've made to the query. I am sure PostgreSQL is able to push it down, since it is much faster now. The problem I have is that this is a query generated by an ORM, So I can't change it.

I would like to understand why wasn't Postgres able to optimize it to a nested loop. Is there something I can do with the statistics?

I don't think. Postgres optimizer just doesn't support this optimization. It has sense only when you know so number of loops is very small - else nested loop should be much slower.

Regards

Pavel



Thanks!!


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