Re: Performance difference in accessing differrent columns in a Postgres Table - Mailing list pgsql-performance

From Tom Lane
Subject Re: Performance difference in accessing differrent columns in a Postgres Table
Date
Msg-id 16506.1532905259@sss.pgh.pa.us
Whole thread Raw
In response to Re: Performance difference in accessing differrent columns in aPostgres Table  (David Rowley <david.rowley@2ndquadrant.com>)
Responses Re: Performance difference in accessing differrent columns in aPostgres Table  (Pavel Stehule <pavel.stehule@gmail.com>)
List pgsql-performance
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


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