From: | Reece Hart <reece(at)harts(dot)net> |
---|---|
To: | pgsql-general <pgsql-general(at)postgresql(dot)org> |
Cc: | tom(at)tacocat(dot)net |
Subject: | Re: Random Sample |
Date: | 2007-05-18 21:16:13 |
Message-ID: | 1179522973.6910.26.camel@snafu.site |
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Thread: | |
Lists: | pgsql-general |
On Fri, 2007-05-18 at 15:36 -0500, tom(at)tacocat(dot)net wrote:
> How do I pull a random sample of either 100 records or 5% of the
> population of a table?
If you can be a little flexible about the number of samples, you can try
select * from table where random()<=0.05;
Of course, there's nothing that guarantees that you'll get 5% and this
only works reasonably for large N. On the other hand, if N were small,
you probably wouldn't be asking for a random sample.
You could also try
select * from table order by random() limit 100;
That'll be expensive, but get you exactly 100 (if your table has >= 100
rows, of course).
-Reece
--
Reece Hart, http://harts.net/reece/, GPG:0x25EC91A0
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