I believe based on semi-recent posts that MIN and MAX are now treated
as special cases in 8.1, and are synonymous with select id order by id
desc limit 1 etc..
Alex
On 10/24/05, Douglas McNaught <doug@xxxxxxxxxxxx> wrote:
> felix@xxxxxxxxxxx writes:
>
> > However, in the process of investigating this, my boss found something
> > which we do not understand. A table with a primary key 'id' takes 200
> > seconds to SELECT MAX(id), but is as close to instantaneous as you'd
> > want for SELECT ID ORDER BY ID DESC LIMIT 1. I understand why
> > count(*) has to traverse all records, but why does MAX have to? This
> > table has about 750,000 rows, rather puny.
>
> As I understand it, because aggregates in PG are extensible (the query
> planner just knows it's calling some function), MAX isn't specially
> handled--the planner doesn't know it's equivalent to the other query.
>
> There has been some talk of special-casing this, but I'm not sure
> where it lead--you might check the archives.
>
> -Doug
>
> ---------------------------(end of broadcast)---------------------------
> TIP 2: Don't 'kill -9' the postmaster
>
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TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match
