Thank you Laurenz and Tom! I'm going to quote Tom's email here:
On 9/17/25 16:41, Tom Lane wrote:
=?UTF-8?Q?Fr=C3=A9d=C3=A9ric_Yhuel?= <frederic.yhuel@xxxxxxxxxx> writes:
Hello, in the following, I don't understand why:
1) the expression index isn't used in the first EXPLAIN
The planner doesn't look for multi-clause matches of that sort.
You could apply a little ju-jitsu perhaps:
regression=# EXPLAIN (ANALYZE, SUMMARY OFF, BUFFERS OFF) SELECT * FROM foo WHERE (ackid IS NULL AND crit = 'WARNING') is true;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------
Index Scan using foo_expr_idx on foo (cost=0.29..8.39 rows=5 width=17) (actual time=0.013..0.016 rows=5.00 loops=1)
Index Cond: (((ackid IS NULL) AND (crit = 'WARNING'::text)) = true)
Index Searches: 1
(3 rows)
Thanks, it works well indeed.
but my own tendency would be to use a partial index rather than a
boolean-valued index:
regression=# CREATE INDEX foo_partial_idx ON foo (id) WHERE ackid IS NULL AND crit = 'WARNING';
CREATE INDEX
regression=# EXPLAIN (ANALYZE, SUMMARY OFF, BUFFERS OFF) SELECT * FROM foo WHERE ackid IS NULL AND crit = 'WARNING';
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Index Scan using foo_partial_idx on foo (cost=0.13..107.18 rows=990 width=17) (actual time=0.010..0.014 rows=5.00 loops=1)
Index Searches: 1
(2 rows)
The advantage of a partial index is you might be able to have the
index entries themselves carry some other column(s), allowing
more queries to be made into index-only scans. I put "id" here,
which might or might not be of any use in this specific toy example.
Yes, Laurenz made a similar suggestion, but the problem is that I'm
mostly interested in the estimated number of output rows... because in
the real query, there's a very bad Hash Join above (the Nested Loop is
*much* faster).
2) the number of estimated rows is completely off in the second EXPLAIN,
whereas the planner could easily use the statistics of foo_f_idx.
Hmm, not sure about that. Again, boolean-valued indexes aren't
something we've worked on too hard, but I don't see why that
would affect this case.
OK, thanks anyway, I think the ju-jitsu mentioned above will do, even
though the application code will have to be patched.
