select * from R
where  a1 op1 C1 and ... and an opn Cn 

The one with best selectivity for a_i op_i C_i   (i.e. fewest matching tuples/pages)

Factors determining selectivity of a_i op_i C_i

• tend to assume uniform distribution of values in dom(a_i)
• equality test on primary key gives at most one match
• equality test on larger dom(a_i) ⇒ less matches
• range test over large part of dom(a_i) is not helpful