組合索引：最左優先原則

如：組合索引(col_a,col_b,col_c)

使用到索引的情況如下：

col_a = "some value"

col_a = "some value" and col_b = "some value"

col_a = "some value" and col_c = "some value"

col_a = "some value" and col_b = "some value" and col_c = "some value"

col_a = "some value" and col_c = "some value" and col_b = "some value"

col_b = "some value" and col_a = "some value" and col_c = "some value"

col_b = "some value" and col_c = "some value" and col_a = "some value"

col_c = "some value" and col_a = "some value" and col_b = "some value"

col_c = "some value" and col_b = "some value" and col_a = "some value"

未使用索引的情況

col_b = "some value"

col_c = "some value"

col_b = "some value" and col_c = "some value"

col_c = "some value" and col_b = "some value"

總結，組合索引從最左開始組合，只要包含第一列的查詢都會用到該組合索引

為什么組合索引遵循最左優先原則？

索引方法是Btree，樹狀的，搜索時需要從根節點出發，上層節點對應靠左的值，所以有最左優先原則。