| From: | Peter Eisentraut <peter_e(at)gmx(dot)net> |
|---|---|
| To: | Petr Jelinek <petr(at)2ndquadrant(dot)com>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> |
| Cc: | Simon Riggs <simon(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Jaime Casanova <jaime(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Tomas Vondra <tv(at)fuzzy(dot)cz> |
| Subject: | Re: TABLESAMPLE patch |
| Date: | 2015-04-10 19:26:35 |
| Message-ID: | 552823EB.5090408@gmx.net |
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| Lists: | pgsql-hackers |
On 4/9/15 8:58 PM, Petr Jelinek wrote:
> Well, you can have two approaches to this, either allow some specific
> set of keywords that can be used to specify limit, or you let sampling
> methods interpret parameters, I believe the latter is more flexible.
> There is nothing stopping somebody writing sampling method which takes
> limit as number of rows, or anything else.
>
> Also for example for BERNOULLI to work correctly you'd need to convert
> the number of rows to fraction of table anyway (and that's exactly what
> the one database which has this feature does internally) and then it's
> no different than passing (SELECT 100/reltuples*number_of_rows FROM
> tablename) as a parameter.
What is your intended use case for this feature? I know that "give me
100 random rows from this table quickly" is a common use case, but
that's kind of cumbersome if you need to apply formulas like that. I'm
not sure what the use of a percentage is. Presumably, the main use of
this features is on large tables. But then you might not even know how
large it really is, and even saying 0.1% might be more than you wanted
to handle.
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