Re: Random()

On 11/18/06, A. Kretschmer <andreas(dot)kretschmer(at)schollglas(dot)com> 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
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>
> ---------------------------(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
>