Help Center/ GaussDB/ Developer Guide(Distributed_8.x)/ SQL Optimization/ Hint-based Optimization/ Hint Errors, Conflicts, and Other Warnings
Updated on 2024-08-20 GMT+08:00
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Hint Errors, Conflicts, and Other Warnings

Plan hints change an execution plan. You can use EXPLAIN to view the changes.

Hints containing errors are invalid and do not affect statement execution. The errors will be displayed in different ways based on statement types. Hint errors in an EXPLAIN statement are displayed as a warning on the API. Hint errors in other statements will be recorded in debug1-level logs containing the PLANHINT keyword.

Hint error types are as follows:

  • Syntax errors

    An error will be reported if the syntax tree fails to be reduced. The No. of the row generating an error is displayed in the error details.

    For example, the hint keyword is incorrect, no table or only one table is specified in the leading or join hint, or no tables are specified in other hints. The parsing of a hint is terminated immediately after a syntax error is detected. Only the hints that have been parsed successfully are valid.

    For example:

    1
    		 
    leading((t1 t2)) nestloop(t1) rows(t1 t2 #10)

    The syntax of nestloop(t1) is wrong and its parsing is terminated. Only leading(t1 t2) that has been successfully parsed before nestloop(t1) is valid.

  • Semantic errors
    • An error will be reported if the specified tables do not exist, multiple tables are found based on the hint setting, or a table is used more than once in the leading or join hint.
    • An error will be reported if the index specified in a scan hint does not exist.
    • If multiple tables with the same name exist after a subquery is pulled up and some of them need to be hinted, add aliases for them to avoid name duplication.
  • Duplicated or conflicted hints

    If hint duplication or conflicts occur, only the first hint takes effect. A message will be displayed to describe the situation.

    • Hint duplication indicates that a hint is used more than once in the same query, for example, nestloop(t1 t2) nestloop(t1 t2).
    • A hint conflict indicates that the functions of two hints with the same table list conflict with each other.
      For example, if nestloop (t1 t2) hashjoin (t1 t2) is used, hashjoin (t1 t2) becomes invalid. nestloop(t1 t2) does not conflict with no mergejoin(t1 t2).

      The table list in the leading hint is disassembled. For example, leading ((t1 t2 t3)) will be disassembled as leading((t1 t2)) leading(((t1 t2) t3)), which will conflict with leading((t2 t1)) (if any). In this case, the latter leading(t2 t1) becomes invalid. If two hints use duplicated table lists and only one of them has the specified outer/inner table, the one without a specified outer/inner table becomes invalid.

      Duplicate hints are allowed. However, for duplicate hints, only the first one is used. For other unused hints, an unused hint warning is reported. Take /*+ expand_sublink expand_sublink */ as an example, database uses only the first expand_sublink hint. Therefore, the unused hint warning is still displayed.

  • A hint becomes invalid after a sublink is pulled up.

    In this case, a message will be displayed. Generally, such invalidation occurs when a sublink contains multiple tables to be joined. After the sublink is pulled up, the tables will not be join members.

  • Unsupported column types
    • Skew hints are specified to optimize redistribution. They will be invalid if their corresponding columns do not support redistribution.
  • Hints are not used.
    • If a hashjoin or mergejoin hint is specified for non-equivalent joins, it will not be used.
    • If an indexscan or indexonlyscan hint is specified for a table that does not have an index, it will not be used.
    • The GSI hint is used for tables that do not contain GSIs.
    • If an IndexScan, IndexOnlyScan or GSI hint is specified for a full-table scan, it will not be used. Generally, index paths are generated only when filter conditions are used on index columns. Indexes are not used during a full table scan.
    • The specified IndexOnlyScan hint is used only when the output column contains only indexes.
    • The GSI hint is used only when queries can be pushed down to GSIs.
    • In equivalent joins, only the joins containing equivalence conditions are valid. Therefore, the leading, join, and rows hints specified for the joins without an equivalence condition will not be used. For example, t1, t2, and t3 are to be joined, and the join between t1 and t3 does not contain an equivalence condition. In this case, leading(t1 t3) will not be used.
    • When a stream plan is generated, if the distribution key of the table is the same as that of the join column, the redistribute plan is not generated. If they are different and the distribution key of another table is the same as that of the join column, only the redistribute plan can be generated, and the broadcast plan cannot be generated. If the corresponding hint is specified, the plan will not be used.
    • If no sublink is pulled up, the specified blockname hint will not be used.
    • Skew hints are not used possibly because:
      • The plan does not require redistribution.
      • The columns specified by hints contain distribution keys.
      • Skew information specified in hints is incorrect or incomplete, for example, no value is specified for join optimization.
      • Skew optimization is disabled by GUC parameters.
Parent topic: Hint-based Optimization