On Tue, Jan 14, 2020 at 04:21:57PM -0500, Tom Lane wrote:
Tomas Vondra <tomas.von...@2ndquadrant.com> writes:
On Tue, Jan 14, 2020 at 03:12:21PM -0500, Tom Lane wrote:
cc'ing Tomas in case he has any thoughts about it.
Well, I certainly do thoughts about this - it's pretty much exactly what
I proposed yesterday in this thread:
https://www.postgresql.org/message-id/flat/20200113230008.g67iyk4cs3xbnjju@development
The third part of that patch series is exactly about supporting extended
statistics on expressions, about the way you described here. The current
status of the WIP patch is that grammar + ANALYZE mostly works, but
there is no support in the planner. It's obviously still very hackish.
Cool. We should probably take the discussion to that thread, then.
I'm also wondering if we could/should 100% rely on extended statistics,
because those are really meant to track correlations between columns,
Yeah, it seems likely to me that the infrastructure for this would be
somewhat different --- the user-facing syntax could be basically the
same, but ultimately we want to generate entries in pg_statistic not
pg_statistic_ext_data. Or at least entries that look the same as what
you could find in pg_statistic.
Yeah. I think we could invent a new type of statistics "expressions"
which would simply built this per-column stats. So for example
CREATE STATISTICS s (expressions) ON (a*b), sqrt(c) FROM t;
would build per-column stats stored in pg_statistics, while
CREATE STATISTICS s (mcv) ON (a*b), sqrt(c) FROM t;
would build the multi-column MCV list on expressions.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
