On Wed, 17 Mar 2021 at 19:07, Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
> On 3/17/21 7:54 PM, Dean Rasheed wrote:
> >
> > it might have been better to estimate the first case as
> >
> > ndistinct((a+b)) * ndistinct(c) * ndistinct(d)
> >
> > and the second case as
> >
> > ndistinct((a+b)) * ndistinct((c+d))
>
> OK. I might be confused, but isn't that what the algorithm currently
> does? Or am I just confused about what the first/second case refers to?
>
No, it currently estimates the first case as ndistinct((a+b),c) *
ndistinct(d). Having said that, maybe that's OK after all. It at least
makes an effort to account for any correlation between (a+b) and
(c+d), using the known correlation between (a+b) and c. For reference,
here is the test case I was using (which isn't really very good for
catching dependence between columns):
DROP TABLE IF EXISTS foo;
CREATE TABLE foo (a int, b int, c int, d int);
INSERT INTO foo SELECT x%10, x%11, x%12, x%13 FROM generate_series(1,100000) x;
SELECT COUNT(DISTINCT a) FROM foo; -- 10
SELECT COUNT(DISTINCT b) FROM foo; -- 11
SELECT COUNT(DISTINCT c) FROM foo; -- 12
SELECT COUNT(DISTINCT d) FROM foo; -- 13
SELECT COUNT(DISTINCT (a+b)) FROM foo; -- 20
SELECT COUNT(DISTINCT (c+d)) FROM foo; -- 24
SELECT COUNT(DISTINCT ((a+b),c)) FROM foo; -- 228
SELECT COUNT(DISTINCT ((a+b),(c+d))) FROM foo; -- 478
-- 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 = 2964, Actual = 478
-- This estimate is ndistinct((a+b),c) * ndistinct(d) = 228*13
-- 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 = 480, Actual = 478
-- This estimate is ndistinct((a+b)) * ndistinct((c+d)) = 20*24
I think that's probably pretty reasonable behaviour, given incomplete
stats (the estimate with no extended stats is capped at 10000).
Regards,
Dean
