Holger Marzen <holger(at)marzen(dot)de> writes:
> You're right. When the result is small, the index is used. When the
> number of rows is a little bit higher and postgres uses a table scan,
> the query is much slower. The table is not very unordered. Can I force
> postgres to raise the limit where it starts scanning the whole table?
Not directly. There is code in 7.2 to try to take account of the
effects of table ordering, but it probably needs to be improved.
See cost_index in costsize.c if you are interested.
regards, tom lane