Re: Hash Join over Nested Loop - Mailing list pgsql-performance

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

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  |