Re: FETCH FIRST clause PERCENT option

On Tue, Mar 26, 2019 at 03:06:52PM +0300, Surafel Temesgen wrote:
>On Thu, Feb 28, 2019 at 11:16 PM Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
>wrote:
>
>>
>> To give you a (admittedly, somewhat contrived and artificial example):
>>
>> SELECT * FROM t1 WHERE id IN (
>> SELECT id FROM t2 ORDER BY x FETCH FIRST 10 PERCENT ROWS ONLY
>> );
>>
>> Maybe this example is bogus and/or does not really matter in practice. I
>> don't know, but I've been unable to convince myself that's the case.
>
>
>does this means we abandon incremental approach? and am not sure of
>calculating
>percentage after OFFSET clause is acceptable or not
>

I don't follow. I don't think I've suggested to abandon the incremental
approach - I've explained why I think it's what the patch should be doing,
and illustrated that with an example. I admit the example may be too
artificial and we never (or very infrequently) generate such plans. But
no such argument was presented here.

As for the OFFSET, I don't see why that would be incompatible with PERCENT
clause. I'm not sure if it should compute the percentage from the total
number of rows (including those skipped by the OFFSET) or the rest, but I
assume SQL standard says something about that.

cheers

--
Tomas Vondra http://www.2ndQuadrant.com
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