| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Surafel Temesgen <surafel3000(at)gmail(dot)com> |
| Cc: | PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: FETCH FIRST clause PERCENT option |
| Date: | 2018-08-16 14:34:21 |
| Message-ID: | 20180816143421.rxla4tonz2dx5xez@alap3.anarazel.de |
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| Thread: | |
| Lists: | pgsql-hackers |
Hi,
On 2018-08-16 17:27:45 +0300, Surafel Temesgen wrote:
> FETCH FIRST with PERCENT option is SQL standard that state limit count to
> be specified in a percentage in addition to specify it in exact count and
> listed as one of Major features simply not implemented yet in recent wiki
> page [1].
>
> I implemented it by executing the query plan twice. One without limit
> filter to find the total number of row that will be feed to limit node so
> the exact limit count can be calculated and the query plan execute again
> with limit filter with newly calculated exact count .
Won't that have rather massive issues with multiple evaluations of
clauses in the query? Besides being really expensive?
I think you'd have to instead spill the query results into a tuplestore
(like we do for FOR HOLD queries at end of xact), and then do the
computations based on that.
- Andres
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