Re: select max(field) from table much faster with a group by clause?

Palle Girgensohn <girgen@pingpong.net> writes:
> When running
>  select max("when") from login where userid='userid'
> it takes a second or two, but when adding "group by userid" the planner
> decides on using another plan, and it gets *much* faster. See example below.

It's only faster for cases where there are few or no rows for the
particular userid ...

> Number of tuples per user varies from zero to a couple of thousands.

The planner is using an intermediate estimate of 406 rows.  You might be
well advised to increase the statistics target for login.userid --- with
luck that would help it to choose the right plan type for both common
and uncommon userids.

            regards, tom lane