Note that the index variable ( {i in TOY} ) comes before the :.
注意索引变量（{iinTOY}）都是在 :之前出现的。
For an index key column that is of variable length.
具有可变长度的索引键列。
A value index measures general changes in the total value of some variable.
价值指数用以度量某些变量的价值总量变化。
A value index measures general changes in the total value of some variable.
价值指数用以度量某些变量的价值总量变化。
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