Configuration Parameter Hints

Function

A hint, or a GUC hint, specifies a configuration parameter value when a plan is generated. Currently, only the following parameters are supported:

Syntax

set [global](guc_name guc_value)

Parameters

• global indicates that the parameter set by hint takes effect at the statement level. If global is not specified, the parameter takes effect only in the subquery where the hint is located.
• guc_name indicates the name of the configuration parameter specified by hint.

• guc_value indicates the value of a configuration parameter specified by hint.

• If a parameter set by hint takes effect at the statement level, the hint must be written to the top-level query instead of the subquery. For UNION, INTERSECT, EXCEPT, and MINUS statements, you can write the GUC hint at the statement level to any SELECT clause that participates in the set operation. The configuration parameters set by the GUC hint take effect on each SELECT clause that participates in the set operation.
• When a subquery is pulled up, all GUC hints on the subquery are discarded.
• The enable_fast_query_shipping, enable_nodegroup_debug, expected_computing_nodegroup, query_dop, and rewrite_rule parameters can be set only at the statement level.
• If a parameter is set by both the statement-level GUC hint and the subquery-level GUC hint, the subquery-level GUC hint takes effect in the corresponding subquery, and the statement-level GUC hint takes effect in other subqueries of the statement.

Example

Hint the query plan in Examples as follows:

explain
select /*+ set global(query_dop 0) */ i_product_name product_name
...

This hint indicates that the query_dop parameter is set to 0 when the plan for a statement is generated, which means the SMP adaptation function is enabled. The generated plan is as follows: