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  1. MariaDB Server
  2. MDEV-10037

UNION with LIMIT ROWS EXAMINED does not require parentheses

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    Details

    • Type: Bug
    • Status: Closed (View Workflow)
    • Priority: Major
    • Resolution: Fixed
    • Affects Version/s: 10.1, 10.2
    • Fix Version/s: 10.2.1
    • Component/s: Parser
    • Labels:
      None

      Description

      10.1 disallowed use of non-parenthesized UNION parts that have ORDER or LIMIT clauses, according to the SQL Standard. That was a merge from MySQL-5.6.

      This script:

      DROP TABLE IF EXISTS t1;
      CREATE TABLE t1 (a INT);
      INSERT INTO t1 VALUES (10),(10);
      SELECT * FROM t1 LIMIT 1
      UNION
      SELECT * FROM t1 LIMIT 1;
      

      returns an error starting from 10.1:

      ERROR 1221 (HY000): Incorrect usage of UNION and LIMIT
      

      However, if I use LIMIT in combination with ROWS EXAMINED:

      SELECT * FROM t1 LIMIT ROWS EXAMINED 1
      UNION
      SELECT * FROM t1 LIMIT ROWS EXAMINED 1;
      

      it returns a result instead of an error:

      +------+
      | a    |
      +------+
      |   10 |
      +------+
      1 row in set, 1 warning (0.00 sec)
      

      It should be fixed to require parentheses as well, like in case of a "normal" LIMIT clause.

      Most likely, LIMIT ROWS EXAMINED was just forgotten during the merge.

      The documentation at:
      https://mariadb.com/kb/en/mariadb/union/
      says:

      Individual selects can contain their own ORDER BY and LIMIT clauses. In that case, the individual queries need to be wrapped between parenthesis.

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              Assignee:
              bar Alexander Barkov
              Reporter:
              bar Alexander Barkov
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                Dates

                Created:
                Updated:
                Resolved: