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
