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
The item I'm focused on is node 23. The estimate is for 7 rows, actual is 896 (multiplied by 1062 loops). I'm confused
abouttwo 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 constraints
totheir respective tables (zulu_five.five_uniform and juliet_india.whiskey_mike). The planner can know in advance that
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) that
satisfythe 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?
Thanks,
Philip
