Re: Unexpected sort order.

Jeff Davis <pgsql(at)j-davis(dot)com> writes:
> On Mon, 2006-11-27 at 12:44 -0800, Ron Mayer wrote:
>> Shouldn't the results of this query shown here been sorted by "b" rather than by "a"?

>> li=# select * from (select (random()*10)::int as a, (random()*10)::int as b from generate_series(1,10) order by a) as x order by b;

> It looks like a planner bug.

It looks to me like the planner thinks that order by a and order by b
are equivalent because the expressions are equal(); hence it discards
what it thinks is a redundant second sort step.

I suppose we could add a check for whether the sort expression contains
volatile functions before believing this, but I'm having a hard time
believing that there are any real-world cases where the check wouldn't
be a waste of cycles. What's the use-case for sorting by a volatile
expression in the first place?

regards, tom lane