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Join Order Hints

Updated at:Jul 15, 2020 GMT+08:00

Function

Theses hints specify the join order and outer/inner tables.

Syntax

  • Specify only the join order.
1
leading(join_table_list) 
  • Specify the join order and outer/inner tables. The outer/inner tables are specified by the outermost parentheses.
1
leading((join_table_list)) 

Parameter Description

join_table_list specifies the tables to be joined. The values can be table names or table aliases. If a subquery is pulled up, the value can also be the subquery alias. Separate the values with spaces. You can add parentheses to specify the join priorities of tables.

A table name or alias can only be a string without a schema name.

An alias (if any) is used to represent a table.

To prevent semantic errors, tables in the list must meet the following requirements:

  • The tables must exist in the query or its subquery to be pulled up.
  • The table names must be unique in the query or subquery to be pulled up. If they are not, their aliases must be unique.
  • A table appears only once in the list.
  • An alias (if any) is used to represent a table.

For example:

leading(t1 t2 t3 t4 t5): t1, t2, t3, t4, and t5 are joined. The join order and outer/inner tables are not specified.

leading(t1 t2 t3 t4 t5): t1, t2, t3, t4, and t5 are joined in sequence. The table on the right is used as the inner table in each join.

leading(t1 (t2 t3 t4) t5): First, t2, t3, and t4 are joined and the outer/inner tables are not specified. Then, the result is joined with t1 and t5, and the outer/inner tables are not specified.

leading(t1 (t2 t3 t4) t5): First, t2, t3, and t4 are joined and the outer/inner tables are not specified. Then, the result is joined with t1, and (t2 t3 t4) is used as the inner table. Finally, the result is joined with t5, and t5 is used as the inner table.

leading((t1 (t2 t3) t4 t5)) leading((t3 t2)): First, t2 and t3 are joined and t2 is used as the inner table. Then, the result is joined with t1, and (t2 t3) is used as the inner table. Finally, the result is joined with t4 and then t5, and the table on the right in each join is used as the inner table.

Examples

Hint the query plan in Examples as follows:

1
2
explain
select /*+ leading((((((store_sales store) promotion) item) customer) ad2) store_returns) leading((store store_sales))*/ i_product_name product_name ...

First, store_sales and store are joined and store_sales is the inner table. Then, The result is joined with promotion, item, customer, ad2, and store_returns in sequence. The optimized plan is as follows:

For details about the warning at the top of the plan, see Hint Errors, Conflicts, and Other Warnings.

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