Help Center/ GaussDB/ Developer Guide(Centralized_2.x)/ Performance Tuning/ SQL Optimization/ Hint-based Tuning/ Sublink Name Hints
Updated on 2023-10-23 GMT+08:00
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Sublink Name Hints

Function

These hints specify the name of a sublink block.

Syntax

1
 
blockname (table)

Parameter Description

  • table specifies the name you have specified for a sublink block.
  • The blockname hint is used by an outer query only when the corresponding sublink is not pulled up. Currently, only the Agg equivalent join, IN, and EXISTS sublinks can be pulled up. This hint is usually used together with the hints described in the previous sections.
  • The subquery after the FROM keyword is hinted by using the subquery alias. In this case, blockname becomes invalid.
  • If a sublink contains multiple tables, the tables will be joined with the outer-query tables in a random sequence after the sublink is pulled up. In this case, blockname also becomes invalid.

Example

1
 
explain select /*+nestloop(store_sales tt) */ * from store_sales where ss_item_sk in (select /*+blockname(tt)*/ i_item_sk from item group by 1);

tt indicates the sublink block name. After being pulled up, the sublink is joined with the outer-query table store_sales by using nestloop. The optimized plan is as follows:

Parent topic: Hint-based Tuning