Re: [HACKERS] What about LIMIT in SELECT ?

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> > Thomas is correct on this.  Vadim has run some tests, and with our
> > optimized psort() code, the in-memory sort is often faster than using
> > the index to get the tuple, because you are jumping all over the drive.
> > I don't remember, but obviously there is a break-even point where
> > getting X rows using the index on a table of Y rows is faster , but
> > getting X+1 rows on a table of Y rows is faster getting all the rows
> > sequentailly, and doing the sort.
> >
> > You would have to pick only certain queries(no joins, index matches
> > ORDER BY), take the number of rows requested, and the number of rows
> > selected, and figure out if it is faster to use the index, or a
> > sequential scan and do the ORDER BY yourself.
>
> Since a sort loads the data into memory anyway, how about speeding up the
> sort by using the index? Or does that take up too much memory? (approx 40%
> more than the data alone, I think)

Not sure you can do that.  The index points to heap tuples/tids, and
though there are tids in the rows, you can't access them as tids in
memory.


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