Hi
I have discovered an issue on my Postgresql database recently installed : it
seems that the optimizer can not, when possible, simplify and rewrite a simple
query before running it. Here is a simple and reproducible example :
my_db=# create table test (n numeric);
CREATE
my_db=# insert into test values (1); --> run 10 times
INSERT
my_db=# insert into test values (0); --> run 10 times
INSERT
my_db=# select count(*) from test;
count
-------
20
(1 row)
my_db=# vacuum full analyze test;
VACUUM
my_db=# explain select * from test where n = 1;
QUERY PLAN
------------------------------------------------------
Seq Scan on test (cost=0.00..1.25 rows=10 width=9)
Filter: (n = 1::numeric)
(2 rows)
my_db=# explain select * from test where n = 1 and n = 1;
QUERY PLAN
-----------------------------------------------------
Seq Scan on test (cost=0.00..1.30 rows=5 width=9)
Filter: ((n = 1::numeric) AND (n = 1::numeric))
(2 rows)
In the first SELECT query (with "where n=1"), the estimated number of returned
rows is correct (10), whereas in the second SELECT query (with "where n=1 and
n=1"), the estimated number of returned rows is 5 (instead of 10 !)
So the optimizer has under-estimated the number of rows returned
That issue is very annoying because with generated SQL queries (from Business
Objects for example) on big tables, it is possible that some queries have
several times the same "where" condition ("where n=1 and n=1" for example), and
as the optimizer is under-estimating the number of returned rows, some bad
execution plans can be chosen (nested loops instead of hash joins for example)
Is the estimated number of returned rows directly linked to the decision of the
optimizer to chose Hash Joins or Nested Loops in join queries ?
Is there a way for the Postgresql optimizer to be able to simplify and rewrite
the SQL statements before running them ? Are there some parameters that could
change the execution plans ?
Thanks by advance for your help
Jean-Francois SURANTYN
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