From: Jay Levitt <jay.levitt@gmail.com>
To: pgsql-performance@postgresql.org
Subject: Predicates not getting pushed into SQL function?
Date: Thu, 03 Nov 2011 13:47:55 -0400
Message-ID: <4EB2D3CB.7020708@gmail.com> (raw)
I'm confused. I have a now-trivial SQL function that, unrestricted, would
scan my whole users table. When I paste the body of the function as a
subquery and restrict it to one row, it only produces one row. When I paste
the body of the function into a view and restrict it to one row, it produces
one row. But when I put it in a SQL function... it scans the whole users
table and then throws the other rows away.
I thought SQL functions were generally inline-able, push-down-able, etc. As
a workaround, I can put my WHERE clause inside the function and pass it
parameters, but that feels ugly, and it won't help for things like
resticting via JOINs. The real function needs parameters, so I can't use it
as a view. Are there better workarounds?
I suspect the problem is (something like) the planner doesn't realize the
function will produce a variable number of rows; I can specify COST or ROWS,
but they're both fixed values.
Pretty-printed function and explain analyze results:
https://gist.github.com/1336963
In ASCII for web-haters and posterity:
-- THE OVERLY SIMPLIFIED FUNCTION
create or replace function matcher()
returns table(user_id int, match int) as $$
select o.user_id, 1 as match
from (
select u.id as user_id, u.gender
from users as u
) as o
cross join
(
select u.id as user_id, u.gender
from users as u
where u.id = 1
) as my;
$$ language sql stable;
-- WHEN I CALL IT AS A FUNCTION
select * from matcher() where user_id = 2;
LOG: duration: 1.242 ms plan:
Query Text:
select o.user_id, 1 as match
from (
select u.id as user_id, u.gender
from users as u
) as o
cross join
(
select u.id as user_id, u.gender
from users as u
where u.id = 1
) as my;
Nested Loop (cost=0.00..118.39 rows=1656 width=4) (actual
time=0.022..0.888 rows=1613 loops=1)
Output: u.id, 1
-> Index Scan using users_pkey on public.users u (cost=0.00..8.27
rows=1 width=0) (actual time=0.013..0.015 rows=1 loops=1)
Index Cond: (u.id = 1)
-> Seq Scan on public.users u (cost=0.00..93.56 rows=1656 width=4)
(actual time=0.004..0.479 rows=1613 loops=1)
Output: u.id
CONTEXT: SQL function "matcher" statement 1
LOG: duration: 1.951 ms plan:
Query Text: select * from matcher() where user_id = 2;
Function Scan on public.matcher (cost=0.25..12.75 rows=5 width=8)
(actual time=1.687..1.940 rows=1 loops=1)
Output: user_id, match
Filter: (matcher.user_id = 2)
-- WHEN I CALL IT AS A SUBQUERY
select * from
(
select o.user_id, 1 as match
from (
select u.id as user_id, u.gender
from users as u
) as o
cross join
(
select u.id as user_id, u.gender
from users as u
where u.id = 1
) as my
) as matcher
where user_id = 2;
LOG: duration: 0.044 ms plan:
Query Text: select * from
(
select o.user_id, 1 as match
from (
select u.id as user_id, u.gender
from users as u
) as o
cross join
(
select u.id as user_id, u.gender
from users as u
where u.id = 1
) as my
) as matcher
where user_id = 2;
Nested Loop (cost=0.00..16.55 rows=1 width=4) (actual
time=0.028..0.031 rows=1 loops=1)
Output: u.id, 1
-> Index Scan using users_pkey on public.users u (cost=0.00..8.27
rows=1 width=4) (actual time=0.021..0.022 rows=1 loops=1)
Output: u.id
Index Cond: (u.id = 2)
-> Index Scan using users_pkey on public.users u (cost=0.00..8.27
rows=1 width=0) (actual time=0.004..0.006 rows=1 loops=1)
Index Cond: (u.id = 1)
-- WHEN I CALL IT AS A VIEW
create view matchview as
select o.user_id, 1 as match
from (
select u.id as user_id, u.gender
from users as u
) as o
cross join
(
select u.id as user_id, u.gender
from users as u
where u.id = 1
) as my;
select * from matchview where user_id = 2;
LOG: duration: 0.044 ms plan:
Query Text: select * from matchview where user_id = 2;
Nested Loop (cost=0.00..16.55 rows=1 width=4) (actual
time=0.028..0.031 rows=1 loops=1)
Output: u.id, 1
-> Index Scan using users_pkey on public.users u (cost=0.00..8.27
rows=1 width=4) (actual time=0.021..0.022 rows=1 loops=1)
Output: u.id
Index Cond: (u.id = 2)
-> Index Scan using users_pkey on public.users u (cost=0.00..8.27
rows=1 width=0) (actual time=0.005..0.007 rows=1 loops=1)
Index Cond: (u.id = 1)
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To: pgsql-performance@postgresql.org
Cc: jay.levitt@gmail.com
Subject: Re: Predicates not getting pushed into SQL function?
In-Reply-To: <4EB2D3CB.7020708@gmail.com>
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