On Mon, 2 Feb 2004, Tomasz Myrta wrote:
> Dnia 2004-02-02 19:30, U¿ytkownik scott.marlowe napisa³:
> > Not entirely, since it only has to sort two columns, it will be smaller,
> > and will therefore be somewhat faster.
>
> Can you say something more about it? Will it be enough faster to keep
> them both? Did anyone make such tests?
that really depends on the distribution of the third column. If there's
only a couple of values in the third column, no big deal. If each entry
is unique, and it's a large table, very big deal.
It is only useful to have a three column index if you actually use it. If
you have an index on (a,b,c) and select order by b, the index won't get
used unless the a part is in the where clause.
the other issue is updates. IT WILL cost more to update two indexes
rather than one. Generally, you can drop / readd the index and use
explain analyze on one of your own queries to see if that helps.
