http://www.qaix.com/postgresql-database-development/246-557-select-based-on-function-result-read.shtml
A more simple testcase is below. Adding OFFSET 0 to the inner query does indeed fix it in my case.
SELECT tmp.user_id AS foo, tmp.user_id AS bar, tmp.user_id AS baz FROM ( SELECT u.user_id FROM users u ) AS tmp;
Seq Scan on users (cost=0.00..1.53 rows=53 width=4) (actual time=0.230..0.233 rows=1 loops=1)
Total runtime: 0.272 ms
---------------------------
SELECT tmp.user_id AS foo, tmp.user_id AS bar, tmp.user_id AS baz FROM ( SELECT (SELECT 1) AS user_id FROM users u ) AS tmp;
Seq Scan on users u (cost=0.03..1.56 rows=53 width=0) (actual time=0.216..0.219 rows=1 loops=1)
InitPlan
-> Result (cost=0.00..0.01 rows=1 width=0) (actual time=0.004..0.006 rows=1 loops=1)
-> Result (cost=0.00..0.01 rows=1 width=0) (actual time=0.002..0.004 rows=1 loops=1)
-> Result (cost=0.00..0.01 rows=1 width=0) (actual time=0.002..0.003 rows=1 loops=1)
Total runtime: 0.270 ms
---------------------------
SELECT tmp.user_id AS foo, tmp.user_id AS bar, tmp.user_id AS baz FROM ( SELECT (SELECT 1) AS user_id FROM users u OFFSET 0 ) AS tmp;
Subquery Scan tmp (cost=0.01..1.03 rows=1 width=4) (actual time=0.032..0.042 rows=1 loops=1)
-> Limit (cost=0.01..1.02 rows=1 width=0) (actual time=0.026..0.033 rows=1 loops=1)
InitPlan
-> Result (cost=0.00..0.01 rows=1 width=0) (actual time=0.003..0.004 rows=1 loops=1)
-> Seq Scan on users u (cost=0.00..1.01 rows=1 width=0) (actual time=0.022..0.027 rows=1 loops=1)
Total runtime: 0.090 ms
On 04.04.2005 17:18, Hannes Dorbath wrote:
Mhh. I have no clue about the internals of PostgreSQL and query planing, but to me as user this should really be a thing the optimizer has to work out..
On 03.04.2005 10:01, PFC wrote:
Noticed this problem,too.
You can always make the calculation you want done once inside a set returning function so it'll behave like a table, but that's ugly.
On Mon, 28 Mar 2005 16:14:44 +0200, Hannes Dorbath <[EMAIL PROTECTED]> wrote:
hm, a few days and not a single reply :|
any more information needed? test data? simplified test case? anything?
thanks
Hannes Dorbath wrote:
The query and the corresponding EXPLAIN is at
http://hannes.imos.net/query.txt
I'd like to use the column q.replaced_serials for multiple calculations
in the SELECT clause, but every time it is referenced there in some way
the whole query in the FROM clause returning q is executed again.
This doesn't make sense to me at all and eats performance.
If this wasn't clear enough, for every
q.replaced_serials <insert_random_calculation> AS some_column
in the SELECT clause there is new block of
---------------------------------------------------------------
-> Aggregate (cost=884.23..884.23 rows=1 width=0)
-> Nested Loop (cost=0.00..884.23 rows=1 width=0)
-> Index Scan using ix_rma_ticket_serials_replace on
rma_ticket_serials rts (cost=0.00..122.35
rows=190 width=4)
Index Cond: ("replace" = false)
-> Index Scan using pk_serials on serials s
(cost=0.00..3.51 rows=1 width=4)
Index Cond: (s.serial_id = "outer".serial_id)
Filter: ((article_no = $0) AND (delivery_id = $1))
---------------------------------------------------------------
in the EXPLAIN result.
For those who wonder why I do this FROM (SELECT...). I was searching for
a way to use the result of an subselect for multiple calculations in the
SELECT clause and return that calculation results as individual columns.
I tested a bit further and found out that PG behaves the same in case q
is a view. This makes me wonder how efficient the optimizer can work
with views - or even worse - nested views.
Tested and reproduced on PG 7.4.1 linux and 8.0.0 win32.
Thanks in advance,
Hannes Dorbath
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