Bruce Momjian wrote:
>Greg Stark wrote:
>
>
>>It has nothing to do with MVCC. It has to do with implementing this is hard in
>>the general case.
>>
>>Think of examples like:
>>
>>select max(foo) group by bar;
>>
>>or
>>
>>select max(foo) where xyz = z;
>>
>>To do it properly max/min have to be special-cased and tightly integrated with
>>other code to handle index scans and aggregates. As it currently stands
>>they're implemented the same way as any other aggregate, which means they get
>>to see all the records in the grouping.
>>
>>This is a frequently asked question, I'm surprised you didn't find stuff
>>searching with google. There have been numerous long discussions on this topic
>>not long ago. People are still trying to think about how to handle this
>>better.
>>
>>
>
>The FAQ does have the example of using ORDER BY LIMIT 1 for MAX(). What
>we don't have a workaround for is COUNT(*). I think that will require
>some cached value that obeys MVCC rules of visibility.
>
>
IMHO portability is an important point. People are used to MAX() and
COUNT(*), and will be surprised that they need some special treatment.
While the reasons for this are perfectly explainable, speeding up these
aggregates with some extra effort would make porting a bit easier.
Regards,
Andreas