转：什么是index的leading column（索引的前导列）？
A leading portion of an index is a set of one or more columns that were specified first and consecutively in the list of columns in the CREATE INDEX statement that created the index. Consider this CREATE INDEX statement:
CREATE INDEX comp_ind ON table1(x, y, z);
x, xy, and xyz combinations of columns are leading portions of the index
yz, y, and z combinations of columns are not leading portions of the index
