On Sat, Nov 09, 2019 at 12:33:05PM -0800, Mark Dilger wrote:
On 11/6/19 11:58 AM, Tomas Vondra wrote:
On Wed, Nov 06, 2019 at 08:54:40PM +0100, Tomas Vondra wrote:
On Mon, Oct 28, 2019 at 04:20:48PM +0100, Tomas Vondra wrote:
Hi,
PostgreSQL 10 introduced extended statistics, allowing us to consider
correlation between columns to improve estimates, and PostgreSQL 12
added support for MCV statistics. But we still had the limitation that
we only allowed using a single extended statistics per relation, i.e.
given a table with two extended stats
CREATE TABLE t (a int, b int, c int, d int);
CREATE STATISTICS s1 (mcv) ON a, b FROM t;
CREATE STATISTICS s2 (mcv) ON c, d FROM t;
and a query
SELECT * FROM t WHERE a = 1 AND b = 1 AND c = 1 AND d = 1;
we only ever used one of the statistics (and we considered them in a not
particularly well determined order).
This patch addresses this by using as many extended stats as possible,
by adding a loop to statext_mcv_clauselist_selectivity(). In each step
we pick the "best" applicable statistics (in the sense of covering the
most attributes) and factor it into the oveall estimate.
Tomas,
Your patch compiles and passes the regression tests for me on debian
linux under master.
Thanks.
Since your patch does not include modified regression tests, I wrote a
test that I expected to improve under this new code, but running it
both before and after applying your patch, there is no change. Please
find the modified test attached. Am I wrong to expect some change in
this test's output? If so, can you provide a test example that works
differently under your patch?
Those queries are not improved by the patch, because we only support
clauses "Var op Const" for now - your tests are using "Var op Var" so
that doesn't work.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
