什麼是index的leading column（索引的前導列）？

看到經常有人問此問題！doc的原話，看到了隨手記錄下！

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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