A JOIN B与B JOIN A [英] A JOIN B vs B JOIN A

问题描述

当处理大型数据集时，表A与行B相比，行数要少得多.将表 A 与 B 和 B 和下面的查询中的 A

when dealing with large datasets where Table A has considerably very less no of rows when compared to B. Is there any difference between joining Table A with B and B with A in the query below

从 A 内联接 B 中选择A.col_PK，B.col_PK A.col_PK = A.col_PK WHERE A.col_2 ="0001";

SELECT A.col_PK , B.col_PK FROM A INNER JOIN B ON A.col_PK = A.col_PK WHERE A.col_2 = "0001";

推荐答案

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对于INNER JOIN，功能上没有区别(顺序确实对外部联接有所影响).为了提高性能，优化器通常选择最佳顺序.

For an INNER JOIN, there is no difference in functionality (order does make a difference for outer joins). For performance, the optimizer generally chooses the best ordering.

在某些情况下，当查询具有较长的连接序列时，优化器可能无法考虑所有选项.在这种情况下，排序可以改变性能.

Under some circumstances, when your query has long sequences of joins, the optimizer may not be able to consider all options. In those cases, the ordering could make a difference to the performance.

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