From: | Andres Freund <andres(at)2ndquadrant(dot)com> |
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To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Peter Geoghegan <pg(at)heroku(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>, Stephen Frost <sfrost(at)snowman(dot)net> |
Subject: | Re: Performance optimization of btree binary search |
Date: | 2013-12-05 14:18:17 |
Message-ID: | 20131205141817.GH14419@alap2.anarazel.de |
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Lists: | pgsql-hackers |
On 2013-12-05 08:58:55 -0500, Tom Lane wrote:
> Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> > I don't think we can get rid of that dance in record_image_eq - it very
> > well could used on records originally generated when those bits haven't
> > been guaranteed to be zeroed.
>
> No, you're failing to think about the context here.
Ah yes. I was completely forgetting that
heap_form_tuple()/heap_fill_tuple() will properly take care to only use
meaningful parts of the (to-be-)stored data, not random padding.
Thanks.
> The risk we take by simplifying comparisons in a more general context
> is that some function somewhere might've been sloppy about doing a
> native-type-to-Datum conversion on its result. In the case of V0
> functions that risk is unavoidable except by adding some explicit cleanup
> code, but I'm a bit worried that somebody, particularly third-party code,
> might've sloppily written "return foo" in a V1 function when "return
> Int32GetDatum(foo)" would be correct. In that case, the resultant Datum
> might have not-per-spec high-order bits, and if it reaches the fast
> comparator without ever having been squeezed into a physical tuple,
> we've got a problem.
Too bad V1 hasn't insisted on using PG_RETURN_* macros. That would have
allowed asserts checking against such cases by setting
fcinfo->has_returned = true or such...
Greetings,
Andres Freund
--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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