Re: [HACKERS] Solution for LIMIT cost estimation

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От Chris Bitmead
Тема Re: [HACKERS] Solution for LIMIT cost estimation
Дата
Msg-id 38A3B2F7.286CD6ED@nimrod.itg.telecom.com.au
обсуждение исходный текст
Ответ на Solution for LIMIT cost estimation  (Tom Lane <tgl@sss.pgh.pa.us>)
Ответы Re: [HACKERS] Solution for LIMIT cost estimation
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Don Baccus wrote:

> But ... that doesn't mean that some folks might not want to use
> it differently.  What if LIMIT 2 were more efficient that COUNT(*)
> in order to determine if more than one row satisfies a condition?

select count(*) > 1 from a;

And if that's not efficient, why not optimise _that_, since it 
expresses directly what you want?

> But I wouldn't feel badly at all if LIMIT limited to queries
> with ORDER BY.  I think this could be done gramatically, i.e.
> 
> [query] ORDER BY

If you are going to limit it thus, it only makes sense if you
either order by a unique key or order by every single column.
Otherwise, why limit it at all? And that can't be determined
gramatically.


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