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  1. MariaDB Server
  2. MDEV-6292

Slow performance with outer joins and join buffer.




      A query over empty tables is extremely slow when outer join is used together with join buffering (which is ON by default).

      Take this file

      drop table if exists t2,t1;
      CREATE TABLE `t1` (
        `id` int(11) NOT NULL AUTO_INCREMENT,
        `col1` varchar(255) NOT NULL DEFAULT '',
        PRIMARY KEY (`id`)
      CREATE TABLE `t2` (
        `id` int(11) NOT NULL AUTO_INCREMENT,
        `parent_id` smallint(3) NOT NULL DEFAULT '0',
        `col2` varchar(25) NOT NULL DEFAULT '',
        PRIMARY KEY (`id`)
      select now();
      SELECT t.* 
        t1 t 
        LEFT JOIN t2 c1 ON c1.parent_id = t.id AND c1.col2 = "val"
        LEFT JOIN t2 c2 ON c2.parent_id = t.id AND c2.col2 = "val"
        LEFT JOIN t2 c3 ON c3.parent_id = t.id AND c3.col2 = "val"
        LEFT JOIN t2 c4 ON c4.parent_id = t.id AND c4.col2 = "val"
        LEFT JOIN t2 c5 ON c5.parent_id = t.id AND c5.col2 = "val"
        LEFT JOIN t2 c6 ON c6.parent_id = t.id AND c6.col2 = "val"
        LEFT JOIN t2 c7 ON c7.parent_id = t.id AND c7.col2 = "val"
        LEFT JOIN t2 c8 ON c8.parent_id = t.id AND c8.col2 = "val"
        LEFT JOIN t2 c9 ON c9.parent_id = t.id AND c9.col2 = "val"
        LEFT JOIN t2 c10 ON c10.parent_id = t.id AND c10.col2 = "val"
        LEFT JOIN t2 c11 ON c11.parent_id = t.id AND c11.col2 = "val"
        LEFT JOIN t2 c12 ON c12.parent_id = t.id AND c12.col2 = "val"
        LEFT JOIN t2 c13 ON c13.parent_id = t.id AND c13.col2 = "val"
        LEFT JOIN t2 c14 ON c14.parent_id = t.id AND c14.col2 = "val"
        LEFT JOIN t2 c15 ON c15.parent_id = t.id AND c15.col2 = "val"
        LEFT JOIN t2 c16 ON c16.parent_id = t.id AND c16.col2 = "val"
        LEFT JOIN t2 c17 ON c17.parent_id = t.id AND c17.col2 = "val"
        LEFT JOIN t2 c18 ON c18.parent_id = t.id AND c18.col2 = "val"
        LEFT JOIN t2 c19 ON c19.parent_id = t.id AND c19.col2 = "val"
        LEFT JOIN t2 c20 ON c20.parent_id = t.id AND c20.col2 = "val"
        LEFT JOIN t2 c21 ON c21.parent_id = t.id AND c21.col2 = "val"
        LEFT JOIN t2 c22 ON c22.parent_id = t.id AND c22.col2 = "val"
        LEFT JOIN t2 c23 ON c23.parent_id = t.id AND c23.col2 = "val"
        LEFT JOIN t2 c24 ON c24.parent_id = t.id AND c24.col2 = "val"
        LEFT JOIN t2 c25 ON c25.parent_id = t.id AND c25.col2 = "val"
        LEFT JOIN t2 c26 ON c26.parent_id = t.id AND c26.col2 = "val"
        LEFT JOIN t2 c27 ON c27.parent_id = t.id AND c27.col2 = "val"
        LEFT JOIN t2 c28 ON c28.parent_id = t.id AND c28.col2 = "val"
        LEFT JOIN t2 c29 ON c29.parent_id = t.id AND c29.col2 = "val"
        LEFT JOIN t2 c30 ON c30.parent_id = t.id AND c30.col2 = "val"
        LEFT JOIN t2 c31 ON c31.parent_id = t.id AND c31.col2 = "val"
        LEFT JOIN t2 c32 ON c32.parent_id = t.id AND c32.col2 = "val"
        LEFT JOIN t2 c33 ON c33.parent_id = t.id AND c33.col2 = "val"
      ORDER BY 
      select now();

      and then run it.
      First, run it with join_cache_level=0 - it will finish instantly
      Then, run it with join_cache_level=2 (the default). The query will take more than 10 minutes (more in debug builds).




            igor Igor Babaev
            psergei Sergei Petrunia
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