On Mon, Jan 2, 2017 at 10:59 AM, Pavan Deolasee
<pavan.deolasee@gmail.com> wrote:
>
> On Mon, Jan 2, 2017 at 10:17 AM, Amit Kapila <amit.kapila16@gmail.com>
> wrote:
>>
>> On Mon, Jan 2, 2017 at 9:28 AM, Pavan Deolasee <pavan.deolasee@gmail.com>
>> wrote:
>> >
>> >
>> > On Mon, Jan 2, 2017 at 8:52 AM, Amit Kapila <amit.kapila16@gmail.com>
>> > wrote:
>> >>
>> >>
>> >> I think there is some chance that such a change could induce
>> >> regression for the cases when there are many index columns or I think
>> >> even when index is on multiple columns (consider index is on first and
>> >> eight column in a ten column table).
>> >>
>> >
>> > I don't see that as a problem because the routine only checks for
>> > columns
>> > that are passed as "interesting_cols".
>> >
>>
>> Right, but now it will evaluate for all interesting_cols whereas
>> previously it would just stop at first if that column is changed.
>>
>
> Ah ok. I read your comment "consider index is on first and
> eight column in a ten column table" as s/eight/eighth. But may be you're
> referring to
> the case where there is an index on eight or nine columns of a ten column
> table.
>
I am talking about both kinds of cases. The scenario where we can see
some performance impact is when there is variable-width column before
the index column (in above context before the eighth column) as there
will be cached offset optimization won't work for such a column.
> You're right. That's an additional cost as Alvaro himself explained in the
> original
> post. But both indirect indexes and WARM needs to know information about all
> modified columns. So assuming either of these patches are going to make it,
> we've to bear that cost.
>
Okay, but I think if we know how much is the additional cost in
average and worst case, then we can take a better call. Also, if we
agree, then doing an update-intensive test on a unlogged table or with
asynchronous commit mode can show us the overhead if there is any.
--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com