Re: Subselect and limit/order?

At 11:46 7/08/00 -0400, Tom Lane wrote:
>Philip Warner <pjw@rhyme.com.au> writes:
>> Just wondering why subselect expressions can not have a limit/order clause,
>
>We could ignore the spec and implement this as an extension, but I'd
>want to see some fairly compelling arguments why it's a good idea.
>(In other words, why do you know better than the designers of SQL?)

Well, it's not *just* me. Both Dec/RDB and SQL/Server implement ORDER BY in
subqueries. SQL/Server insists on a limit statement in this case, whereas
Dec/Rdb just dies if more than one row is returned.

It is a very useful thing when you are scanning though sequential records
that are time based, or have ID's that have 'holes' in the sequence. eg. in
the case of time-based data, you can derive durations.

But it can be done by writing a plpgsql function, so it's not a big issue. 


>> It can obviously be done in two select statements, but I was just wondering
>> if it's an oversight or a planner problem?
>
>I'd say mostly an executor problem, actually.  Nobody's figured out
>where the executor would need to be hacked to support tuple-limits
>applied elsewhere than the top level of a select.

Wouldn't is also have a fair impact on the planner? Or does it always
assume that subselects only return one row?



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