Hi hackers,
The planner currently calls approx_tuple_count() to estimate
hashjointuples and mergejointuples. That makes sense when
joinrestrictinfo contains additional clauses beyond the hash/merge
equality list. But if all join restriction clauses are exactly those
hash/merge clauses, the estimate already computed in
path->jpath.path.rows is usually more accurate (and free).
This patch reuses path->jpath.path.rows in that case and skips
approx_tuple_count().
Regression results
==================
| actual | approx | estimate
---------------------+--------+--------+-------
join.sql q1 | 5 | 1 | 5
join.sql q2 | 10 000 | 1 | 10 000
join.sql q3 | 5 | 5 | 5
join.sql q4 | 5 | 5 | 5
join.sql q5 | 5 | 1 | 5
partition_join q1 | 200 | 1 | 200
partition_join q2 | 42 | 84 | 84
partition_join q3 | 8 | 1 | 8
postgres_fdw | 2001 | 1001 | 2001
select_parallel q1 | 0 | 5 000 | 5 000
updatable_views q1 | 2 | 2 | 423
Two cases get worse: select_parallel.sql and updatable_views.sql.
Looking forward to your feedback!
--
Best regards,
Ilia Evdokimov,
Tantor Labs LLC.
