Configuration Parameter Hints

Function

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

Precautions

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.

Syntax

set [global]([@block_name] 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.
block_name indicates the block name of the statement block. For details, see block_name.
guc_name indicates the name of the configuration parameter specified by hint.

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

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_max_mem	Yes
agg_redistribute_enhancement	Yes
best_agg_plan	Yes
cost_model_version	No
cost_param	No
enable_array_optimization	No
enable_bitmapscan	Yes
enable_broadcast	Yes
enable_csqual_pushdown	No
enable_redistribute	Yes
enable_extrapolation_stats	Yes
enable_fast_query_shipping	No
enable_force_vector_engine	No
enable_hashagg	Yes
enable_hashfilter	No
enable_hashjoin	Yes
enable_index_nestloop	Yes
enable_indexonlyscan	Yes
enable_indexscan	Yes
enable_join_pseudoconst	Yes
enable_mergejoin	Yes
enable_mixedagg	No
enable_nestloop	Yes
enable_nodegroup_debug	No
enable_partition_dynamic_pruning	Yes
enable_seqscan	Yes
enable_sonic_hashagg	No
enable_sonic_hashjoin	Yes
enable_sort	Yes
enable_stream_ctescan	No
enable_tidscan	Yes
enable_value_redistribute	Yes
enable_vector_engine	No
expected_computing_nodegroup	No
force_bitmapand	Yes
from_collapse_limit	Yes
join_collapse_limit	Yes
join_num_distinct	Yes
outer_join_max_rows_multipler	Yes
prefer_hashjoin_path	No
qrw_inlist2join_optmode	Yes
qual_num_distinct	Yes
query_dop	No
query_max_mem	No
query_mem	No
rewrite_rule	No
setop_optmode	Yes
skew_option	Yes
stream_ctescan_max_estimate_mem	No
stream_ctescan_pred_threshold	No
stream_ctescan_refcount_threshold	No
windowagg_pushdown_enhancement	No
index_selectivity_cost	Yes
index_cost_limit	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:

Click to enlarge