Updated on 2024-11-05 GMT+08:00

Configuration Parameter Hints

Function

A hint, or a GUC hint, specifies a configuration parameter value when a plan is generated.

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.
  • 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.

Currently, GUC hints support only some configuration parameters. Some parameters cannot be configured at the subquery level and can only be configured at the statement level. The following table lists the supported parameters.

Table 1 Configuration parameters supported by GUC hints

Parameter

Configured at the Subquery Level (Yes/No)

agg_redistribute_enhancement

Yes

best_agg_plan

Yes

cost_model_version

No

cost_param

No

enable_bitmapscan

Yes

enable_broadcast

Yes

enable_extrapolation_stats

Yes

enable_fast_query_shipping

No

enable_force_vector_engine

No

enable_hashagg

Yes

enable_hashjoin

Yes

enable_index_nestloop

Yes

enable_indexscan

Yes

enable_join_pseudoconst

Yes

enable_nestloop

Yes

enable_nodegroup_debug

No

enable_partition_dynamic_pruning

Yes

enable_sort

Yes

enable_vector_engine

No

expected_computing_nodegroup

No

force_bitmapand

Yes

from_collapse_limit

Yes

join_collapse_limit

Yes

join_num_distinct

Yes

qrw_inlist2join_optmode

Yes

qual_num_distinct

Yes

query_dop

No

rewrite_rule

No

skew_option

Yes

Examples

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: