As previously stated, ordered input is essential to a merge scan join, but it is not required for nested loop joins.
如前所述,按顺序的输入对于合并扫描联接是必不可少的,但其无需嵌套循环联接。
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Typically, the optimizer chooses merge scan join when the cardinality of the outer input is not obviously small and there is no efficient access method for the inner table.