From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <david.rowley@2ndquadrant.com>
cc: Dinesh Kumar <dns98944@gmail.com>,
        postgres performance list <pgsql-performance@postgresql.org>
Subject: Re: Performance difference in accessing differrent columns in a
 Postgres Table
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Comments: In-reply-to David Rowley <david.rowley@2ndquadrant.com>
	message dated "Mon, 30 Jul 2018 09:53:55 +1200"
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Date: Sun, 29 Jul 2018 19:00:59 -0400
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David Rowley <david.rowley@2ndquadrant.com> writes:
> On 29 July 2018 at 17:38, Dinesh Kumar <dns98944@gmail.com> wrote:
>> I found performance variance between accessing int1 and int200 column which
>> is quite large.

> Have a look at slot_deform_tuple and heap_deform_tuple. You'll see
> that tuples are deformed starting at the first attribute. If you ask
> for attribute 200 then it must deform 1-199 first.

Note that that can be optimized away in some cases, though evidently
not the one the OP is testing.  From memory, you need a tuple that
contains no nulls, and all the columns to the left of the target
column have to be fixed-width datatypes.  Otherwise, the offset to
the target column is uncertain, and we have to search for it.

			regards, tom lane