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  1. MariaDB Server
  2. MDEV-10447

different execution plan for single- and multi-DELETE

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    Description

      create table t1 (id int);
      		 
      insert t1 values (1),(2),(3);
      		 
      create table t2 (id int, acc_id int);
      		 
      insert t2 values (1,1),(2,2),(3,3);
      		 
      explain delete from t1 where id in (select id from t2 where acc_id = 9999);
      		 
      explain delete uc from t1 as uc where uc. id in (select p. id from t2 p where p.acc_id = 9999);

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          relates to

          Task - A task that needs to be done. MDEV-7487 Semi-join optimization for single-table update/delete statements

          • Critical - Crashes, loss of data, severe memory leak.
          • Closed

          Activity

            rhyscampbell Rhys Campbell added a comment -

            Just an example a little closer to what I saw. Version 10.0.24..

            CREATE TABLE t1 (id INT PRIMARY KEY);
            		 
            INSERT t1 VALUES (1),(2),(3);
            		 
            CREATE TABLE t2 (id INT PRIMARY KEY, acc_id INT, KEY(acc_id));
            		 
            INSERT t2 VALUES (1,1),(2,2),(3,3);
            		 
            EXPLAIN DELETE FROM t1 WHERE id IN (SELECT id FROM t2 WHERE acc_id = 9999); # Table scan first!
            		 
            EXPLAIN DELETE uc FROM t1 AS uc WHERE uc.id IN (SELECT p.id FROM t2 p WHERE p.acc_id = 9999); # the plan we want
            		 
             
            		 
            EXPLAIN DELETE FROM t1 WHERE t1.id IN (SELECT t2.id FROM t2 WHERE t2.acc_id = 9999) RETURNING t1.id; # Even aliasing like this produceds the poor plan

            rhyscampbell Rhys Campbell added a comment - Just an example a little closer to what I saw. Version 10.0.24.. CREATE TABLE t1 (id INT PRIMARY KEY); INSERT t1 VALUES (1),(2),(3); CREATE TABLE t2 (id INT PRIMARY KEY, acc_id INT, KEY(acc_id)); INSERT t2 VALUES (1,1),(2,2),(3,3); EXPLAIN DELETE FROM t1 WHERE id IN (SELECT id FROM t2 WHERE acc_id = 9999); # Table scan first! EXPLAIN DELETE uc FROM t1 AS uc WHERE uc.id IN (SELECT p.id FROM t2 p WHERE p.acc_id = 9999); # the plan we want   EXPLAIN DELETE FROM t1 WHERE t1.id IN (SELECT t2.id FROM t2 WHERE t2.acc_id = 9999) RETURNING t1.id; # Even aliasing like this produceds the poor plan

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              Unassigned Unassigned
              serg Sergei Golubchik
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