On 2 Feb 2018 15:06, "Laurenz Albe" <laurenz.a...@cybertec.at> wrote:
>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
>them rather than using the first 25 >results it gets by scanning then
>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