On Mon, Oct 26, 2020 at 12:50:38PM -0400, Philip Semanchuk wrote:
> I'm trying to understand a bad estimate by the planner, and what I can do about it. The anonymized plan is here:
https://explain.depesz.com/s/0MDz
What postgres version ?
Since 9.6(?) FKs affect estimates.
> The item I'm focused on is node 23. The estimate is for 7 rows, actual is 896 (multiplied by 1062 loops). I'm
confusedabout two things in this node.
>
> The first is Postgres' estimate. The condition for this index scan contains three expressions --
>
> (five_uniform = zulu_five.five_uniform) AND
> (whiskey_mike = juliet_india.whiskey_mike) AND
> (bravo = 'mike'::text)
>
> The columns in the first two expressions (five_uniform and whiskey_mike) are NOT NULL, and have foreign key
constraintsto their respective tables (zulu_five.five_uniform and juliet_india.whiskey_mike). The planner can know in
advancethat 100% of the rows in the table will satisfy those criteria.
>
> For the third expression (bravo = 'mike'), Postgres has excellent statistics. The estimated frequency of 'mike' is
2.228%,actual frequency is 2.242%, so Postgres' estimate is only off by a tiny amount (0.014%).
>
> From what I understand, the planner has all the information it needs to make a very accurate estimate here, but it's
offby quite a lot. What information am I failing to give to the planner?
>
> My second point of confusion is related. There are 564,071 rows in the source table (xray_india, aliased as papa)
thatsatisfy the condition bravo = 'mike'. EXPLAIN reports the actual number of rows returned as 896*1062 ~= 951,552. I
understandthat the number reported by EXPLAIN is often a bit bigger, but this discrepancy is much larger than I'm
expecting.What am I missing here?
