The WHERE clause is used to filter the rows specified by
the table sources in the FROM clause. The expression
in the WHERE clause is a boolean expression; any rows
for which the boolean expression evaluates to true are returned.
WHERE expressions also can occur in UPDATE
and DELETE statements.
When a SELECT expression operates against a base
table, QED attempts to use an index to speed up the execution of the
statement, based on the terms that appear in the WHERE
clause.
QED tries to find an index or unique constraint that matches
an item in the where clause.
