Re: Query optimiser is not using 'not null' constraint when 'order bynulls last' clause is used
| От | Nandakumar M |
|---|---|
| Тема | Re: Query optimiser is not using 'not null' constraint when 'order bynulls last' clause is used |
| Дата | |
| Msg-id | CANcFUu4YyAFYUYw2P=UGTJNo4TKFqSMOjnx5pi1apxiuTqeR+g@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Query optimiser is not using 'not null' constraint when 'orderby nulls last' clause is used (Laurenz Albe <laurenz.albe@cybertec.at>) |
| Ответы |
Re: Query optimiser is not using 'not null' constraint when 'order by nulls last' clause is used
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| Список | pgsql-performance |
Hi,
On 2 Feb 2018 15:06, "Laurenz Albe" <laurenz.albe@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
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