From: Tom Lane <tgl@sss.pgh.pa.us>
To: Simon Riggs <simon@2ndquadrant.com>
cc: "Patrik VV." <paddor@gmail.com>, pgsql-docs@postgresql.org
Subject: Re: Wording in TABLESAMPLE documentation
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Comments: In-reply-to Simon Riggs <simon@2ndquadrant.com>
	message dated "Fri, 12 Aug 2016 18:50:57 +0100"
Date: Fri, 12 Aug 2016 13:54:54 -0400
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Simon Riggs <simon@2ndquadrant.com> writes:
> On 12 August 2016 at 16:23, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I think "global" might have implications we don't want.  How about
>> adding ", based on a system-generated seed"?

> What I was trying to express was that

> SELECT setseed(dp);
> SELECT * FROM foo TABLESAMPLE ...;
> SELECT * FROM foo TABLESAMPLE ...;
> SELECT * FROM foo TABLESAMPLE ...;

> would yield a repeatable set of samples, similarly repeatable but not
> same samples as

> SELECT * FROM foo TABLESAMPLE ... REPEATABLE;
> SELECT * FROM foo TABLESAMPLE ... REPEATABLE;
> SELECT * FROM foo TABLESAMPLE ... REPEATABLE;

But that's *wrong*.  Not all tablesample methods make any such guarantee.
In fact, neither of our contrib methods do.  Only if you use REPEATABLE
(and the method allows it) is there any promise at all about repeatability.

			regards, tom lane


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