Re: Index usage in order by with multiple columns in order-by-clause

Andreas Joseph Krogh <andreak(at)officenet(dot)no> writes:
> Is there a way I can have multiple columns in the ORDER BY clause, each with
> different ASC/DESC-order and still use an index to speed up sorting?

A btree index isn't magic, it's just an ordered list of entries. So you
can't just randomly flip the ordering of individual columns. For
instance, the natural sort order of a 2-column index on (x,y) is like

x y

1 1
1 2
1 3
2 1
2 2
2 3
3 1
3 2
3 3

If you scan this index forwards, you get the equivalent of
ORDER BY x ASC, y ASC
If you scan it backwards, you get the equivalent of
ORDER BY x DESC, y DESC
But there is no way to get the equivalent of x ASC, y DESC from
a scan of this index, nor x DESC, y ASC.

If you have a specific requirement for one of those combinations,
what you can do is build an index in which one of the columns is
"reverse sorted". For instance, if we reverse-sort y, the index
ordering looks like

x y

1 3
1 2
1 1
2 3
2 2
2 1
3 3
3 2
3 1

Now we can get ORDER BY x ASC, y DESC from a forwards indexscan,
or ORDER BY x DESC, y ASC from a backwards scan. But there's no
way to get ASC/ASC or DESC/DESC from this index. If you really need
all four orderings to be available, you're stuck with maintaining
two indexes.

Reverse-sorted index columns are possible but not well supported in
existing PG releases (you need a custom operator class, and the planner
is not all that bright about using them). 8.3 will have full support.

regards, tom lane