Selectivity from hard-wired assumptions

Selectivity from hard-wired assumptions 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. Selectivityhard-wired assumptions

These assumptions are shown in the following table.

Selectivity for various operations for index scans when search values are unknown in advance and statistics are not usedThis table shows the 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)