Message-ID: <553EA399.1010508@BlueTreble.com>
Date: Mon, 27 Apr 2015 16:01:13 -0500
From: Jim Nasby <Jim.Nasby@BlueTreble.com>
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To: Amit Kapila <amit.kapila16@gmail.com>
CC: Petr Jelinek <petr@2ndquadrant.com>, Andres Freund <andres@anarazel.de>,
 <hlinnaka@iki.fi>, Robert Haas <robertmhaas@gmail.com>, Bruce Momjian
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 <stark@mit.edu>, PostgreSQL-development <pgsql-hackers@postgresql.org>, "Jeff
 Janes" <jeff.janes@gmail.com>
Subject: Re: Reducing tuple overhead
References: <20150423162429.GG3055@alap3.anarazel.de>
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On 4/25/15 12:12 AM, Amit Kapila wrote:
>  > ... which isn't possible. You can not go from a heap tuple to an
> index tuple.
>
> We will have the access to index value during delete, so why do you
> think that we need linkage between heap and index tuple to perform
> Delete operation?  I think we need to think more to design Delete
> .. by CTID, but that should be doable.

The problem with just having the value is that if *anything* changes 
between how you evaluated the value when you created the index tuple and 
when you evaluate it a second time you'll corrupt your index. This is 
actually an incredibly easy problem to have; witness how we allowed 
indexing timestamptz::date until very recently. That was clearly broken, 
but because we never attempted to re-run the index expression to do 
vacuuming at least we never corrupted the index itself.

>  > This has been discussed in the past.
>
> I have tried to search in archive, but not getting what is the exact
> problem.

Unfortunately I can't find prior discussion now either... :/
-- 
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com


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