On 11/18/06, A. Kretschmer <[EMAIL PROTECTED]> wrote:
am Sat, dem 18.11.2006, um 23:02:33 +0530 mailte Rajesh Kumar Mallah folgendes: > >select <bigint_column> from <your_table> order by random() limit 1; > > This query will tend to get slower as the table grows because of the > sorting. Right. > > it possible to get a row from a random offset > how about > > select <bigint_column> from <your_table> limit 1 offset 20000*random() ; For instance, you have a table with only 500 rows and random() returns 0.999? In other words: in think, this is a bad idea, unless you use a constant for the offset-calculation thats not larger than the number of rows in this table. For this, you need this value, but select count(1) from table force a seqscan -> slow.
but order by random() would also forces a seqscan , looks like the random offset method performs better explain analyze select count(*) from general.profile_master; +--------------------------------------------------------------------------------------------------------------------------------+ | QUERY PLAN | +--------------------------------------------------------------------------------------------------------------------------------+ | Aggregate (cost=139214.26..139214.27 rows=1 width=0) (actual time= 3071.178..3071.179 rows=1 loops=1) | | -> Seq Scan on profile_master (cost=0.00..137703.21 rows=604421 width=0) (actual time=0.032..2686.842 rows=601240 loops=1) | | Total runtime: 3071.268ms | +--------------------------------------------------------------------------------------------------------------------------------+ (3 rows) tradein_clients=> explain analyze SELECT profile_id from general.profile_master limit 1 offset 601240*random(); +--------------------------------------------------------------------------------------------------------------------------------+ | QUERY PLAN | +--------------------------------------------------------------------------------------------------------------------------------+ | Limit (cost=13770.30..13770.53 rows=1 width=4) (actual time= 1614.146..1614.147 rows=1 loops=1) | | -> Seq Scan on profile_master (cost=0.00..137703.21 rows=604421 width=4) (actual time=0.036..1375.742 rows=429779 loops=1) | | Total runtime: 1614.187ms | +--------------------------------------------------------------------------------------------------------------------------------+ (3 rows) tradein_clients=> explain analyze SELECT profile_id from general.profile_master order by random() limit 1; +--------------------------------------------------------------------------------------------------------------------------------------+ | QUERY PLAN | +--------------------------------------------------------------------------------------------------------------------------------------+ | Limit (cost=207079.39..207079.40 rows=1 width=4) (actual time= 11715.694..11715.695 rows=1 loops=1) | | -> Sort (cost=207079.39..208590.45 rows=604421 width=4) (actual time= 11715.691..11715.691 rows=1 loops=1) | | Sort Key: random() | | -> Seq Scan on profile_master (cost=0.00..139214.26 rows=604421 width=4) (actual time=0.036..4605.259 rows=601241 loops=1) | | Total runtime: 11716.039ms | +--------------------------------------------------------------------------------------------------------------------------------------+ (5 rows) Andreas
-- Andreas Kretschmer Kontakt: Heynitz: 035242/47215, D1: 0160/7141639 (mehr: -> Header) GnuPG-ID: 0x3FFF606C, privat 0x7F4584DA http://wwwkeys.de.pgp.net ---------------------------(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
