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11g对Nested Loops的改进

原创 Oracle 作者:yueli34 时间:2018-11-05 16:37:00 0 删除 编辑

11g官档9 Joins

Current Implementation for Nested Loops Joins

Oracle Database 11 g  introduced a new implementation for nested loops that reduces overall latency for physical I/O. When an index or a table block is not in the buffer cache and is needed to process the join, a physical I/O is required. The database can batch multiple physical I/O requests and process them using a vector I/O (array) instead of one at a time. The database sends an array of rowids to the operating system, which performs the read.

As part of the new implementation, two  NESTED LOOPS  join row sources might appear in the execution plan where only one would have appeared in prior releases. In such cases, Oracle Database allocates one  NESTED LOOPS  join row source to join the values from the table on the outer side of the join with the index on the inner side. A second row source is allocated to join the result of the first join, which includes the rowids stored in the index, with the table on the inner side of the join.

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