| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Dilip Kumar <dilipbalaut(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Proposal: scan key push down to heap [WIP] |
| Date: | 2016-10-26 06:31:39 |
| Message-ID: | 20161026063139.jfzqkta7s3qnhptd@alap3.anarazel.de |
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| Lists: | pgsql-hackers |
On 2016-10-25 13:18:47 -0400, Tom Lane wrote:
> (Frankly, I'm pretty skeptical of this entire patch being worth the
> trouble...)
The gains are quite noticeable in some cases. So if we can make it work
without noticeable downsides...
What I'm worried about though is that this, afaics, will quite
noticeably *increase* total cost in cases with a noticeable number of
columns and a not that selective qual. The reason for that being that
HeapKeyTest() uses heap_getattr(), whereas upper layers use
slot_getattr(). The latter "caches" repeated deforms, the former
doesn't... That'll lead to deforming being essentially done twice, and
it's quite often already a major cost of query processing.
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