sql - Oracle:为什么不使用并行执行？

Question

查看以下查询:如果我注释子查询，它会使用并行执行，否则不会。

查询得到后

SELECT  /*+ parallel(c, 20) */
      1, (SELECT 2 FROM DUAL)
  FROM DUAL c;

Answer 1

您本可以找到答案 in the documentation :

A SELECT statement can be parallelized only if the following conditions are satisfied:

• The query includes a parallel hint specification (PARALLEL or PARALLEL_INDEX) or the schema objects referred to in the query have a PARALLEL declaration associated with them.

• At least one of the tables specified in the query requires one of the following:

  • A full table scan

  • An index range scan spanning multiple partitions

• No scalar subqueries are in the SELECT list.

您的查询处于最后一个障碍:它的投影中有一个标量子查询。如果你想并行化查询，你需要找到另一种方式来编写它。