>In the above case, the optimizer does >not know that it will get the rows >in the correct order: indexes are >sorted ASC NULLS LAST by default, >so a backwards index scan will >produce the results NULLS FIRST, >which is the default for ORDER BY ... >DESC.
The order by column has a not null constraint on it and so nulls last or first shouldn't make any difference.
>If you want the nulls last, PostgreSQL >has to retrieve *all* the rows and sort >them rather than using the first 25 >results it gets by scanning then >indexes.
>To have the above query perform >fast, add additional indexes with either >ASC NULLS FIRST or DESC NULLS >LAST for all used keys.
For now this is exactly what I have done. But it is in effect a duplicate index on a PK column and I would be happy not to create it in the first place.
