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  1. MariaDB Server
  2. MDEV-10842

Wrong results when computing window functions with same sort key but different direction

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      When computing window functions we group them (if possible) by the sort key, so that we perform
      the minimum number of sorts possible. However, the following order by clauses are incompatible, yet we group them together:

      create table t1 (
        pk int primary key,
        a int,
        b int,
        c char(10)
      );
       
      insert into t1 values
      ( 1, 0, 1, 'one'),
      ( 2, 0, 2, 'two'),
      ( 3, 0, 3, 'three'),
      ( 4, 1, 1, 'one'),
      ( 5, 1, 1, 'two'),
      ( 6, 1, 2, 'three'),
      ( 7, 2, NULL, 'n_one'),
      ( 8, 2, 1,    'n_two'),
      ( 9, 2, 2,    'n_three'),
      (10, 2, 0,    'n_four'),
      (11, 2, 10,   NULL);
       
      select row_number() over (order by a), row_number() over (order by a desc) from t;
      select pk,
               row_number() over (order by pk desc) as r_desc,
               row_number() over (order by pk asc) as r_asc
      from t1;
       
      pk	r_desc	r_asc
      1	11	11
      2	10	10
      3	9	9
      4	8	8
      5	7	7
      6	6	6
      7	5	5
      8	4	4
      9	3	3
      10	2	2
      11	1	1
      

      The row numbers should be 11, 10, .. for the first column, but 1, 2, 3, ... for the second column. Running the row_number() queries separately outputs the correct results.
      This should also be considered a potential problem for partition by.

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            Assignee:
            igor Igor Babaev
            Reporter:
            cvicentiu Vicențiu Ciorbaru
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              Updated:
              Resolved:

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