Selection Condition

What
A selection condition is the condition that is used to test tuples when selecting them from a relation

Conjunctive Normal Form
The first step in applying a selection condition is to convert it into conjunctive normal form

original: (day < 8/9/94 AND rname = 'Paul') OR bid=5 OR sid = 3

final: (day < 8/9/94 OR bid=5 OR sid=3) AND (rname = 'Paul' OR bid=5 OR sid=3)

B Tree Index
A B Tree index only matches attributes in the prefix of the search key.

An index of  matches a=5 AND b=3, but not b=3 itself

Hash Index
Searching on a hash index requires that all attributes in the search key be present