Thread: Crazy query plan.
SQL:
CREATE TABLE test (id BIGINT, id2 BIGINT, id3 BIGINT, id4 BIGINT);
INSERT INTO test SELECT i, i, i, i FROM generate_series(0, 9) i;
EXPLAIN ANALYZE SELECT ((SELECT tmp::test FROM (SELECT * FROM test
LIMIT 1) tmp)::test).*;
WILL:
QUERY PLAN
Result (cost=0.11..0.12 rows=1 width=0) (actual time=0.076..0.078
rows=1 loops=1)
InitPlan
-> Subquery Scan tmp (cost=0.00..0.03 rows=1 width=32) (actual
time=0.014..0.019 rows=1 loops=1)
-> Limit (cost=0.00..0.02 rows=1 width=32) (actual
time=0.008..0.009 rows=1 loops=1)
-> Seq Scan on test (cost=0.00..23.10 rows=1310
width=32) (actual time=0.005..0.005 rows=1 loops=1)
-> Subquery Scan tmp (cost=0.00..0.03 rows=1 width=32) (actual
time=0.008..0.013 rows=1 loops=1)
-> Limit (cost=0.00..0.02 rows=1 width=32) (actual
time=0.005..0.006 rows=1 loops=1)
-> Seq Scan on test (cost=0.00..23.10 rows=1310
width=32) (actual time=0.002..0.002 rows=1 loops=1)
-> Subquery Scan tmp (cost=0.00..0.03 rows=1 width=32) (actual
time=0.008..0.012 rows=1 loops=1)
-> Limit (cost=0.00..0.02 rows=1 width=32) (actual
time=0.005..0.006 rows=1 loops=1)
-> Seq Scan on test (cost=0.00..23.10 rows=1310
width=32) (actual time=0.003..0.003 rows=1 loops=1)
-> Subquery Scan tmp (cost=0.00..0.03 rows=1 width=32) (actual
time=0.009..0.013 rows=1 loops=1)
-> Limit (cost=0.00..0.02 rows=1 width=32) (actual
time=0.005..0.006 rows=1 loops=1)
-> Seq Scan on test (cost=0.00..23.10 rows=1310
width=32) (actual time=0.002..0.002 rows=1 loops=1)
Total runtime: 0.138 ms
One subquery for each column. wtf?
On 13/11/2009 7:25 PM, Oleg Serov wrote:
> EXPLAIN ANALYZE SELECT ((SELECT tmp::test FROM (SELECT * FROM test
> LIMIT 1) tmp)::test).*;
This may be simplified to the comparison between these two queries:
SELECT ((SELECT test FROM test LIMIT 1)::test);
SELECT ((SELECT test FROM test LIMIT 1)::test).*;
The former results in a single seq scan in a single subquery:
Result (cost=0.02..0.03 rows=1 width=0)
InitPlan
-> Limit (cost=0.00..0.02 rows=1 width=32)
-> Seq Scan on test (cost=0.00..27.70 rows=1770 width=32)
The latter does this four times:
Result (cost=0.06..0.07 rows=1 width=0)
InitPlan
-> Limit (cost=0.00..0.02 rows=1 width=32)
-> Seq Scan on test (cost=0.00..27.70 rows=1770 width=32)
-> Limit (cost=0.00..0.02 rows=1 width=32)
-> Seq Scan on test (cost=0.00..27.70 rows=1770 width=32)
-> Limit (cost=0.00..0.02 rows=1 width=32)
-> Seq Scan on test (cost=0.00..27.70 rows=1770 width=32)
-> Limit (cost=0.00..0.02 rows=1 width=32)
-> Seq Scan on test (cost=0.00..27.70 rows=1770 width=32)
The change is triggered by expansion of the single-ROW result of the
subquery into a regular 4-tuple.
Is the co0nversion of the ROW into individual fields in the SELECT
clause done by some kind of macro-expansion in parsing/planning?
--
Craig Ringer
> This may be simplified to the comparison between these two queries:
... or even further into:
CREATE TYPE test AS ( a integer, b integer, c integer, d integer );
EXPLAIN SELECT ((SELECT ROW(1,1,1,1)::test)::test);
EXPLAIN SELECT ((SELECT ROW(1,1,1,1)::test)::test).*;
craig=# EXPLAIN SELECT ((SELECT ROW(1,1,1,1)::test)::test);
QUERY PLAN
--------------------------------------------------
Result (cost=0.01..0.02 rows=1 width=0)
InitPlan
-> Result (cost=0.00..0.01 rows=1 width=0)
(3 rows)
craig=# EXPLAIN SELECT ((SELECT ROW(1,1,1,1)::test)::test).*;
QUERY PLAN
--------------------------------------------------
Result (cost=0.04..0.05 rows=1 width=0)
InitPlan
-> Result (cost=0.00..0.01 rows=1 width=0)
-> Result (cost=0.00..0.01 rows=1 width=0)
-> Result (cost=0.00..0.01 rows=1 width=0)
-> Result (cost=0.00..0.01 rows=1 width=0)
(6 rows)
Something odd I stumbled upon while testing for this:
craig=# SELECT tmp FROM (SELECT ((SELECT ROW(1,1,1,1)::test)::test)) AS tmp;
tmp
---------------
("(1,1,1,1)")
(1 row)
... is fine,
craig=# SELECT tmp.* FROM (SELECT ((SELECT ROW(1,1,1,1)::test)::test))
AS tmp;
test
-----------
(1,1,1,1)
(1 row)
... is fine,
craig=# SELECT (tmp.*).* FROM (SELECT ((SELECT
ROW(1,1,1,1)::test)::test)) AS tmp;
test
-----------
(1,1,1,1)
(1 row)
... is ... WTF? How is "(tmp.*).*" the same as "tmp.*" ?
--
Craig Ringer
Sorry for the multiple replies-to-self, but this seemed worth
specifically noting: the expansion also results in multiple calls to
tuple-returning functions, even functions marked VOLATILE. For example:
CREATE OR REPLACE FUNCTION expandtest(INTEGER) RETURNS test AS $$
DECLARE
rec test;
BEGIN
RAISE NOTICE 'expandtest(%)',$1;
rec.a := $1;
rec.b := $1;
rec.c := $1;
rec.d := $1;
RETURN rec;
END;
$$ LANGUAGE 'plpgsql' STRICT VOLATILE;
craig=# SELECT (expandtest(1)).*;
NOTICE: expandtest(1)
NOTICE: expandtest(1)
NOTICE: expandtest(1)
NOTICE: expandtest(1)
a | b | c | d
---+---+---+---
1 | 1 | 1 | 1
(1 row)
I don't know if that's intended behaviour or not, but I certainly find
it very _surprising_ behaviour, especially given the otherwise
equivalent translation:
craig=# SELECT t.* FROM expandtest(1) t;
NOTICE: expandtest(1)
a | b | c | d
---+---+---+---
1 | 1 | 1 | 1
(1 row)