Hi, 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 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
