Обсуждение: Re: [HACKERS] sorting big tables :(

> I have an idea.  Can he run CLUSTER on the data?  If so, the sort will
> not use small batches, and the disk space during sort will be reduced.

I think a real winner would be to use an existing index. This is what others do
to eliminate a sort completely. Of course the optimizer has to choose what is cheaper
on a per query basis (index access or sort of result set).
result set small --> use sort
result set large --> use available index

Andreas

>
>
> > I have an idea.  Can he run CLUSTER on the data?  If so, the sort will
> > not use small batches, and the disk space during sort will be reduced.
>
> I think a real winner would be to use an existing index. This is what others do
> to eliminate a sort completely. Of course the optimizer has to choose what is cheaper
> on a per query basis (index access or sort of result set).
> result set small --> use sort
> result set large --> use available index

Keep in mind an index is going to be seeking all over the table, making
the cache of limited use.  Sometime, when doing a join, the optimizer
chooses a sequential scan rather than use an index for this reason, and
the sequential scan is faster.

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