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  1. MariaDB Server
  2. MDEV-31964

Unexpected performance issue, Join_buffer_space_limit becomes smaller, but the query speed is faster

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Details

    • Bug
    • Status: Open (View Workflow)
    • Major
    • Resolution: Unresolved
    • 11.0.3
    • 11.0(EOL)
    • Optimizer
    • None
    • docker mariadb:11.0

    Description

      When Join_buffer_space_limit is set to the minimum value, the query speed is increased dozens of times, which is unreasonable.

      Steps to reproduce:
      1. Use the mariadb:11.0 container image to create a container, and create a database for the test
      2. Use the SQL statement in init_tables.sql to build a table
      3. Use select_query.sql to query, the query time on my machine is about 9s

      But when performing the same steps 1 and 2, and before performing step 3, SET Session join_buffer_space_limit = 2048;
      The query time will drop to about 0.8s

      Theoretically, `join_buffer_space_limit` is merely an upper limit, and a larger value should result in faster query speeds.

      Attachments

        1. init_tables.sql
          8.88 MB
          fuying
        2. join_buffer_space_limit = 2048.png
          180 kB
          fuying
        3. join_buffer_space_limit = default.png
          165 kB
          fuying
        4. select_query.sql
          3 kB
          fuying

        Activity

          People

            oleg.smirnov Oleg Smirnov
            ying fuying
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            Dates

              Created:
              Updated:

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