| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> |
| Cc: | Dinesh Kumar <dns98944(at)gmail(dot)com>, postgres performance list <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: Performance difference in accessing differrent columns in a Postgres Table |
| Date: | 2018-07-29 23:00:59 |
| Message-ID: | 16506.1532905259@sss.pgh.pa.us |
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| Lists: | pgsql-performance |
David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> writes:
> On 29 July 2018 at 17:38, Dinesh Kumar <dns98944(at)gmail(dot)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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