Tuple filtering is applied for a GROUP BY when the following criteria are met:

• All grouping columns come from the same table and the optimizer has chosen the table in question to be the outermost table in the query block.
• The optimizer has chosen an index as the access path for the table in question.
• The grouping columns, plus any simple column references from the table that have equality predicates on them, are a prefix of the index that the optimizer selected as the access path for the table.

Here is the most common case in which tuple filtering will be applied:

SELECT max(c2) FROM t1 GROUP BY c1

Equality predicates allow tuple filtering on the following:

SELECT c2, SUM(c3)
FROM t1
WHERE c1 = 5 GROUP BY c2

SELECT max(c4)
FROM t1
WHERE c1 = 5 AND c3 = 6 GROUP BY c2