问个index的问题

coolalibaba

在db2的红皮书上看到一段话，不太理解，
assume that there is an index on columns (C1,C2,C3,C4) of the table and that 0 is the lowest value in each column.

1. WHERE C1=5 AND C2=7
Both predicates are stage 1 and the compound predicate is indexable. A matching index scan could be used with C1 and C2 as matching columns.

2. WHERE C1=5 AND C2>7
Both predicates are stage 1 and the compound predicate is indexable. A matching index scan could be used with C1 and C2 as matching columns.

3. WHERE C1>5 AND C2=7
Both predicates are stage 1, but only the first matches the index. A matching index scan could be used with C1 as a matching column.

为啥第3个里面C2不能index呢？

beginner-bj

的确很难理解