In all other cases, uses a fixed number that attempts to describe
the percentage of rows that will probably be returned; it might not correspond
to the actual selectivity of the operation in every case. It is an assumption
hard-wired into the system. These assumptions are shown in .
Selectivity for various operations for index scans
when search values are unknown in advance and statistics are not used
Operator
Selectivity
=, >=, >, <=, <, <> when data type of
parameter is a boolean
.5 (50%)
other operators (except for IS NULL and IS NOT NULL) when
data type of parameter is boolean
.5 (50%)
IS NULL
.1 (10%)
IS NOT NULL
.9 (90%)
=
.1 (10%)
>, >=, <, <=
.33 (3%)
<> compared to non-boolean type
.9 (90%)
LIKE transformed from LIKE predicate (see )
1.0 (100%)
>= and < when transformed internally from LIKE (see )
.25 (.5 X .5)
>= and <= operators when transformed internally from
BETWEEN (see )
.25 (.5 X .5)