Re: Indexes on expressions with multiple columns and operators

On 13/10/2025 16:55, Tom Lane wrote:
> Andrei Lepikhov <lepihov@gmail.com> writes:
>> On 25/9/2025 12:41, Frédéric Yhuel wrote:
>>> So, on SQL Server, you can do this:
>>> CREATE STATISTICS FooStats ON foo (ackid, crit) WHERE crit = 'WARNING';
> 
>> Nice! Thanks for the report. I think the only reason why Postgres
>> doesn't have it yet is the computational cost.
> 
> I think it's more lack of round tuits.  If we had such an option for
> statistics objects, presumably we'd determine the applicability of a
> particular statistics object to a query the same way we do for partial
> indexes, namely try to prove the statistics' restriction condition
> from the query WHERE clauses.  I've not heard complaints about that
> being unduly expensive.
> 
> In the meantime, I believe the old-fashioned approach of creating
> a partial expression index and letting ANALYZE collect stats on that
> will serve, at least for simple statistics.I know at least two extensions (one of which is mine) that attempt to 
analyse query post-execution state, identify unsuccessful predictions on 
cardinality, number of groups, and work_mem, and fix these issues by 
creating MCV and distinct extended statistics.

Of course, without extended statistics on join clauses, their effect is 
highly limited, but we are preparing ;).

Many combinations of clauses may occur. Partial indexes can affect the 
whole system's performance in automatic mode. Additionally, I would 
personally like to play the same game as SQL Server already does - 
compute statistics in an efficient manner - during a Scan. The filter of 
such a scan may serve as a WHERE condition in the extended statistics.

-- 
regards, Andrei Lepikhov,
pgEdge