Re: Query optimiser is not using 'not null' constraint when 'order by nulls last' clause is used

Hi,

On 2 Feb 2018 15:06, "Laurenz Albe" <laurenz(dot)albe(at)cybertec(dot)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
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.

Regards
Nanda