Re: Order by (for 15 rows) adds 30 seconds to query time

"Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov> writes:
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> The approach contemplated in the comment, of assembling some stats
>> on-the-fly from the stats for individual child tables, doesn't
>> seem real practical from a planning-time standpoint.

> Can you give a thumbnail sketch of why that is?

Well, it would be expensive, and it's not even clear you can do it at
all (merging histograms with overlapping bins seems like a mess for
instance).

I think we have previously discussed the idea of generating and storing
ANALYZE stats for a whole inheritance tree, which'd solve the problem
nicely from the planner's standpoint. I'm a bit worried about the
locking implications, but if it took just a SELECT lock on the child
tables it probably wouldn't be too bad --- no worse than any other
SELECT on the inheritance tree. Another thing that's hard to figure out
is how autovacuum would know when to redo the stats. In a lot of common
situations, the inheritance parent table is empty and never changes, so
no autovac or autoanalyze would ever get launched against it.

regards, tom lane