Re: Using index for IS NULL query

От: Tomas Vondra
Тема: Re: Using index for IS NULL query
Дата: ,
Msg-id: 4919F2C6.4000303@fuzzy.cz
(см: обсуждение, исходный текст)
Ответ на: Using index for IS NULL query  ("Andrus")
Ответы: Re: Using index for IS NULL query  ("Vladimir Sitnikov")
Index usage with sub select or inner joins  ("Julien Theulier")
Список: pgsql-performance

Скрыть дерево обсуждения

Using index for IS NULL query  ("Andrus", )
 Re: Using index for IS NULL query  (Tom Lane, )
  Re: Using index for IS NULL query  (Andreas Kretschmer, )
  Re: Using index for IS NULL query  (Matthew Wakeling, )
 Re: Using index for IS NULL query  (Andreas Kretschmer, )
 Re: Using index for IS NULL query  (Tomas Vondra, )
  Re: Using index for IS NULL query  ("Vladimir Sitnikov", )
   Re: Using index for IS NULL query  ("Scott Marlowe", )
   Re: Using index for IS NULL query  (Tomas Vondra, )
  Index usage with sub select or inner joins  ("Julien Theulier", )
   Re: Index usage with sub select or inner joins  (Joshua Tolley, )
    Re: Index usage with sub select or outer joins  ("Julien Theulier", )

> Index is not used for
>
> is null
>
> condition:
>
> create index  makse_dokumnr_idx on makse(dokumnr);
> explain select
>     sum( summa)
>   from MAKSE
>   where  dokumnr is null
>
> "Aggregate  (cost=131927.95..131927.96 rows=1 width=10)"
> "  ->  Seq Scan on makse  (cost=0.00..131927.94 rows=1 width=10)"
> "        Filter: (dokumnr IS NULL)"
 >
 >
> Table makse contains 1200000 rows and about 800 rows with dokumnr is
> null so using index is much faster that seq scan.
> How to fix ?

Yes, NULL values are not stored in the index, but you may create
functional index on

(CASE WHEN dokumnr IS NULL THEN -1 ELSE dokumnr END)

and then use the same expression in the WHERE clause. You may replace
the '-1' value by something that's not used in the dokumnr column.

regards
Tomas


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Сообщение: Re: Using index for IS NULL query