Hint Errors, Conflicts, and Other Warnings

Plan hints change an execution plan. You can run 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 interface. 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:

    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).

      NOTICE:

      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.

  • 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.

  • 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.
    • If an indexscan or indexonlyscan hint is specified for a full-table scan, it will not be used. Generally, index paths are generated only when filtering conditions are used on index columns. Indexes are not used during a full table scan.
    • If an indexonlyscan hint is specified when the output or predicate condition column does not contain only indexes, it will not be used.
    • 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.
    • If no sublink is pulled up, the specified blockname hint will not be used.
    • Skew hints are not used possibly because:
      • 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.
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