Optimizer GUC Parameter Hints

Function

These hints set GUC parameters related to query optimization that take effect during the query execution. For details about the application scenarios of hints, see the description of each GUC parameter.

Syntax

1

set(param value)

Parameter Description

• param indicates the parameter name.
• value indicates the value of a parameter.
• Currently, the following parameters can be set and take effect by using hints:
  • Boolean

    enable_bitmapscan, enable_hashagg, enable_hashjoin, enable_indexscan, enable_indexonlyscan, enable_material, enable_mergejoin, enable_nestloop, enable_index_nestloop, enable_seqscan, enable_sort, enable_tidscan, enable_stream_operator, enable_stream_recursive, enable_broadcast, enable_fast_query_shipping, enable_trigger_shipping, enable_remotejoin, enable_remotegroup, enable_remotelimit, and enable_remotesort

  • Integer

    best_agg_plan and query_dop

  • Floating point

    cost_weight_index, default_limit_rows, seq_page_cost, random_page_cost, cpu_tuple_cost, cpu_index_tuple_cost, cpu_operator_cost, and effective_cache_size

  • Enumerated type

    try_vector_engine_strategy

  • Character string

    node_name

    By setting node_name, you can deliver the current SQL statement to the DN corresponding to node_name for execution.

    Example:

    select /*+ set(node_name datanode1) */ from table_name;

    In the preceding command, datanode1 indicates the name of the DN queried from the pgxc_node system catalog (without quotation marks), and table_name indicates the table name. This query is directly performed on datanode1.

    

    • node_name can be set only by using the SELECT statement. If it is set by using other statements, it does not take effect.
    • node_name can only be set to the name of a DN and cannot be set to the name of a CN.
    • node_name cannot be modified by using the SET statement and can only be used in plan hints.
    • node_name cannot be modified by using gs_guc.
    • node_name supports only simple query statements and does not support complex query statements (such as UNION and UNION ALL), subqueries, and multi-table associations.
    • This operation can be performed by common users.
    • This operation cannot be performed together with row-level access control. If they are performed together, an error will be reported.

• If you set a parameter that is not in the whitelist and the parameter value is invalid or the hint syntax is incorrect, the query execution is not affected. Run explain(verbose on). An error message is displayed, indicating that hint parsing fails.
• The GUC parameter hint takes effect only in the outermost query. That is, the GUC parameter hint in the subquery does not take effect.
• The GUC parameter hint in the view definition does not take effect.
• CREATE TABLE ... AS ... The GUC parameter hint in the outermost query takes effect.