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.
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_bitmapscan |
Yes |
enable_broadcast |
Yes |
enable_redistribute |
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_stream_ctescan |
No |
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 |
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:
