I've noticed that row count estimates for expression indexes appear
to rely on default_statistics_target rather than on a column's
actual statistics target. That is, if I use ALTER TABLE SET
STATISTICS to increase a column's statistics target and then run
ANALYZE, then estimates for non-expression-index queries improve
as expected. However, queries that use an expression index remain
accurate for only around the N most common values, where N is the
default_statistics_target that was in effect when ANALYZE ran. I'm
still rummaging through the archives looking for past discussion;
is this behavior a known limitation or just an oversight?
CREATE TABLE foo (x integer);
CREATE INDEX foo_x_idx ON foo (x);
CREATE INDEX foo_abs_x_idx ON foo (abs(x));
INSERT INTO foo (x)
SELECT r1 % r2
FROM generate_series(1, 100) AS g1(r1),
generate_series(1, 100) AS g2(r2);
SET default_statistics_target TO 15;
ALTER TABLE foo ALTER COLUMN x SET STATISTICS 20;
ANALYZE foo;
SELECT most_common_vals FROM pg_stats WHERE attname = 'x';
most_common_vals
-----------------------------------------------------
{0,1,2,3,4,5,6,7,8,10,9,11,12,14,13,15,16,19,17,18}
(1 row)
EXPLAIN ANALYZE SELECT * FROM foo WHERE x = 13;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on foo (cost=2.72..50.28 rows=205 width=4) (actual
time=0.370..1.766 rows=220 loops=1)
Recheck Cond: (x = 13)
-> Bitmap Index Scan on foo_x_idx (cost=0.00..2.72 rows=205 width=0)
(actual time=0.314..0.314 rows=220 loops=1)
Index Cond: (x = 13)
Total runtime: 2.905 ms
(5 rows)
EXPLAIN ANALYZE SELECT * FROM foo WHERE abs(x) = 13;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on foo (cost=2.72..50.80 rows=205 width=4) (actual
time=0.358..1.720 rows=220 loops=1)
Recheck Cond: (abs(x) = 13)
-> Bitmap Index Scan on foo_abs_x_idx (cost=0.00..2.72 rows=205 width=0)
(actual time=0.305..0.305 rows=220 loops=1)
Index Cond: (abs(x) = 13)
Total runtime: 2.875 ms
(5 rows)
EXPLAIN ANALYZE SELECT * FROM foo WHERE x = 18;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on foo (cost=2.60..49.75 rows=172 width=4) (actual
time=0.312..1.442 rows=180 loops=1)
Recheck Cond: (x = 18)
-> Bitmap Index Scan on foo_x_idx (cost=0.00..2.60 rows=172 width=0)
(actual time=0.262..0.262 rows=180 loops=1)
Index Cond: (x = 18)
Total runtime: 2.393 ms
(5 rows)
EXPLAIN ANALYZE SELECT * FROM foo WHERE abs(x) = 18;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on foo (cost=2.22..43.65 rows=63 width=4) (actual
time=0.313..1.436 rows=180 loops=1)
Recheck Cond: (abs(x) = 18)
-> Bitmap Index Scan on foo_abs_x_idx (cost=0.00..2.22 rows=63 width=0)
(actual time=0.263..0.263 rows=180 loops=1)
Index Cond: (abs(x) = 18)
Total runtime: 2.418 ms
(5 rows)
--
Michael Fuhr
---------------------------(end of broadcast)---------------------------
TIP 2: Don't 'kill -9' the postmaster
