Tilmann Singer wrote:
* andrew@xxxxxxxxxxxx <andrew@xxxxxxxxxxxx> [20070728 21:05]:
Let's try putting the sort/limit in each piece of the UNION to speed them up separately.
SELECT * FROM (
(SELECT * FROM large_table lt
WHERE lt.user_id = 12345
ORDER BY created_at DESC LIMIT 10) AS q1
UNION
(SELECT * FROM large_table lt
WHERE user_id IN (SELECT contact_id FROM relationships WHERE user_id=12345)
ORDER BY created_at DESC LIMIT 10) AS q2
ORDER BY created_at DESC LIMIT 10;
It's not possible to use ORDER BY or LIMIT within unioned queries.
http://www.postgresql.org/docs/8.2/static/sql-select.html#SQL-UNION
"ORDER BY and LIMIT can be attached to a subexpression if it is enclosed in parentheses"
---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match
