实际的类型参数可以由泛型类定义的类型变量匹配。
The actual type arguments can be matched to the type variables defined by the generified class.
对于占用资源很多的sql语句，查看实际的变量值(字符)而不是参数标记也许对优化器较为有益。
For very expensive SQL statements, the optimizer may benefit from seeing actual variable values (literals) instead of parameter markers.
这里，我们将由argno(初始时被设为1以指向命令本身后的第一个实际参数)索引的当前参数复制到字符串变量s1。
Here we copy the current arg, indexed by argno (which was initially set to 1, to point to the first actual arg after the command itself) to string var s1.
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