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Re: Query plan prefers hash join when nested loop is much faster

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On Sat, 22 Aug 2020 at 00:35, iulian dragos
<iulian.dragos@xxxxxxxxxxxxxx> wrote:
> I am trying to understand why the query planner insists on using a hash join, and how to make it choose the better option, which in this case would be a nested loop.

> |                           ->  Index Scan using test_result_module_result_id_idx on test_result  (cost=0.57..6911.17 rows=4331 width=12) (actual time=0.002..0.002 rows=1 loops=14824) |
> |                                 Index Cond: (module_result_id = module_result.id)                                                                                                     |

You might want to check if the pg_stats view reports a realistic
n_distinct value for test_result.module_result_id.  If the
pg_class.retuples is correct for that relation then that would
indicate the n_distinct estimate is about 115000. Going by the number
of rows you've mentioned it would appear a more realistic value for
that would be -0.4. which is 0 - 1 / (500000000 / 200000000.0).
However, that's assuming each module_result  has a test_result.  You
could run a SELECT COUNT(DISTINCT module_result_id) FROM test_result;
to get a better idea.

If ANALYZE is not getting you a good value for n_distinct, then you
can overwrite it. See [1], search for n_distinct.

David

[1] https://www.postgresql.org/docs/current/sql-altertable.html





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