No create statistics on starttime_endtime(distinct), that index on "starttime,endtime", use default analyze, I tested
toincrease statistics_targets but that no help. Could you provide the function name for generic plan selectivity
estimation?
Thanks,
James
-----Original Message-----
From: David Rowley <dgrowleyml@gmail.com>
Sent: Tuesday, December 6, 2022 1:59 PM
To: James Pang (chaolpan) <chaolpan@cisco.com>
Cc: pgsql-performance@lists.postgresql.org; jamespang886@gmail.com
Subject: Re: wrong rows and cost estimation when generic plan
On Tue, 6 Dec 2022 at 18:28, James Pang (chaolpan) <chaolpan@cisco.com> wrote:
> -> Index Scan using idx_xxxxx_time on xxxxx (cost=0.44..8.48 rows=1 width=2923) (actual time=8136.242..8136.242
rows=0loops=1)
> Index Cond: ((starttime = $7) AND (endtime = $8))
> Filter: ((password IS NULL) AND ...(aid = $4) AND (bid = $5) AND (btype = $6) AND...
> Rows Removed by Filter: 5534630
I wonder if you did:
create statistics xxxxx_starttime_endtime_stats (ndistinct) on starttime,endtime from xxxxx; analyze xxxxx;
if the planner would come up with a higher estimate than what it's getting for the above and cause it to use the other
indexinstead.
> optimzer is very complicated, could you direct me how optimizer to do selectivity estimation when building
genericplan, for this case? for custom_plan, optimizer knows boundparams values, but when generic_plan, planner() use
boundparams=NULL, it try to calculate average value based on mcv list of the index attributes (starttime,endtime) ?
IIRC, generic plan estimates become based on distinct estimations rather than histograms or MCVs.
David
