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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