On Wed, 17 Mar 2021 at 20:48, Dean Rasheed <dean.a.rasheed@gmail.com> wrote:
>
> For reference, here is the test case I was using (which isn't really very good for
> catching dependence between columns):
>
And here's a test case with much more dependence between the columns:
DROP TABLE IF EXISTS foo;
CREATE TABLE foo (a int, b int, c int, d int);
INSERT INTO foo SELECT x%2, x%5, x%10, x%15 FROM generate_series(1,100000) x;
SELECT COUNT(DISTINCT a) FROM foo; -- 2
SELECT COUNT(DISTINCT b) FROM foo; -- 5
SELECT COUNT(DISTINCT c) FROM foo; -- 10
SELECT COUNT(DISTINCT d) FROM foo; -- 15
SELECT COUNT(DISTINCT (a+b)) FROM foo; -- 6
SELECT COUNT(DISTINCT (c+d)) FROM foo; -- 20
SELECT COUNT(DISTINCT ((a+b),c)) FROM foo; -- 10
SELECT COUNT(DISTINCT ((a+b),(c+d))) FROM foo; -- 30
-- First case: stats on [(a+b),c]
CREATE STATISTICS s1(ndistinct) ON (a+b),c FROM foo;
ANALYSE foo;
EXPLAIN ANALYSE
SELECT (a+b), (c+d) FROM foo GROUP BY (a+b), (c+d);
-- Estimate = 150, Actual = 30
-- This estimate is ndistinct((a+b),c) * ndistinct(d) = 10*15,
-- which is much better than ndistinct((a+b)) * ndistinct(c) *
ndistinct(d) = 6*10*15 = 900
-- Estimate with no stats = 1500
-- Second case: stats on (c+d) as well
CREATE STATISTICS s2 ON (c+d) FROM foo;
ANALYSE foo;
EXPLAIN ANALYSE
SELECT (a+b), (c+d) FROM foo GROUP BY (a+b), (c+d);
-- Estimate = 120, Actual = 30
-- This estimate is ndistinct((a+b)) * ndistinct((c+d)) = 6*20
Again, I'd say the current behaviour is pretty good.
Regards,
Dean
