Re: Use index to estimate expression selectivity

On 11/23/23 18:30, Tom Lane wrote:
> Bono Stebler <bono.stebler@gmail.com> writes:
>> After discussing the issue on irc, it looks like it could be possible 
>> for the planner to use a partial index matching an expression exactly to 
>> estimate its selectivity.
> 
> I think going forward we're going to be more interested in extending
> CREATE STATISTICS than in adding special behaviors around indexes.
> An index is a pretty expensive thing to maintain if you really only
> want some statistics.  Contrariwise, if you need the index for
> functional reasons (perhaps to enforce some strange uniqueness
> constraint) but you don't like some decision the planner takes because
> of the existence of that index, you're kind of stuck.  So decoupling
> this stuff makes more sense from where I sit.
> 

I agree adding indexes if you only really want the statistics part would
be rather expensive, but I do think using indexes created for functional
reasons as a source of statistics is worth consideration.

Actually, I've been experimenting with using btree indexes to estimate
certain conditions (e.g. the simplest var=const), and the estimates
calculated from internal pages is quite useful. Maybe not as the primary
estimate, but to set "safe" range for non-MCV items. For example if the
traditional estimate says 1 row matches, but we see there's ~100 leaf
pages for that key, maybe we should bump up the estimate ...

But yeah, it may affect the query planning in undesirable ways. Perhaps
we could have "use for statistics" reloption or something ...


regards

-- 
Tomas Vondra
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