Re: Efficiently query for the most recent record for a given user

On Wed, Aug 7, 2013 at 3:34 PM, Tom Lane  wrote:
> Claudio Freire  writes:
>> On Wed, Aug 7, 2013 at 3:12 PM, Robert DiFalco  wrote:
>>> I want to get the last call_activity record for a single user.
>
>> Create an index over (user_id, called desc), and do
>> select * from call_activity where user_id = blarg order by called desc limit 1
>
> Note that there's no particular need to specify "desc" in the index
> definition.  This same index can support searches in either direction
> on the "called" column.


Yeah, but it's faster if it's in the same direction, because the
kernel read-ahead code detects sequential reads, whereas it doesn't
when it goes backwards. The difference can be up to a factor of 10 for
long index scans.

Though... true... for a limit 1... it wouldn't matter that much. But
it's become habit to match index sort order by now.