| From: | Nico Williams <nico(at)cryptonector(dot)com> |
|---|---|
| To: | Vik Fearing <vik(at)postgresfriends(dot)org> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Aleksander Alekseev <aleksander(at)timescale(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Andrei Lepikhov <lepihov(at)gmail(dot)com>, wenhui qiu <qiuwenhuifx(at)gmail(dot)com> |
| Subject: | Re: Should we optimize the `ORDER BY random() LIMIT x` case? |
| Date: | 2025-05-16 21:53:29 |
| Message-ID: | aCez2Uz/yx2DTwPv@ubby |
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| Thread: | |
| Lists: | pgsql-hackers |
On Fri, May 16, 2025 at 11:10:49PM +0200, Vik Fearing wrote:
> Isn't this a job for <fetch first clause>?
>
> Example:
>
> SELECT ...
> FROM ... JOIN ...
> FETCH SAMPLE FIRST 10 ROWS ONLY
>
> Then the nodeLimit could do some sort of reservoir sampling.
The query might return fewer than N rows. What reservoir sampling
requires is this bit of state: the count of input rows so far.
The only way I know of to keep such state in a SQL query is with a
RECURSIVE CTE, but unfortunately that would require unbounded CTE size,
and it would require a way to query next rows one per-iteration.
Nico
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