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  1. MariaDB Server
  2. MDEV-4406

Wrong estimate of records in range for non-nullable column

Details

    • Bug
    • Status: Closed (View Workflow)
    • Major
    • Resolution: Fixed
    • 10.0.1, 10.0.2
    • N/A
    • Optimizer
    • None

    Description

      If a column is defined with NOT NULL specifier then the estimate for the records in range can be greatly off.

      The following test case demonstrates this problem:

      create table t1 (a int not null);
      insert into t1 values
        (7), (6), (4), (9), (1), (5), (2), (1), (3), (8);
       
      set use_stat_tables='preferably';
       
      analyze table t1;
      flush table t1;
       
      set optimizer_use_condition_selectivity=3;
       
      select count(*) from t1 where a between 5 and 7;
      explain extended select * from t1 where a between 5 and 7;

      MariaDB [test]> explain extended select * from t1 where a between 5 and 7;
      +------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
      | id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
      +------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
      |    1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |   10 |    75.00 | Using where |
      +------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+

      If we remove specifier NOT NULL

      alter table t1 change column a a int;

      the estimate becomes good:

      MariaDB [test]> explain extended select * from t1 where a between 5 and 7;
      +------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
      | id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
      +------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
      |    1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |   10 |    25.00 | Using where |
      +------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+

      Attachments

        Activity

          fix idea:

          === modified file 'sql/sql_statistics.cc'
          — sql/sql_statistics.cc 2013-04-16 05:43:07 +0000
          +++ sql/sql_statistics.cc 2013-04-20 14:10:42 +0000
          @@ -3520,7 +3520,7 @@
          {
          double sel, min_mp_pos, max_mp_pos;

          • if (min_endp && !min_endp->key[0])
            + if (min_endp && (!min_endp->key[0] || !field->null_ptr))
            {
            store_key_image_to_rec(field, (uchar *) min_endp->key,
            min_endp->length);
          pomyk Patryk Pomykalski added a comment - fix idea: === modified file 'sql/sql_statistics.cc' — sql/sql_statistics.cc 2013-04-16 05:43:07 +0000 +++ sql/sql_statistics.cc 2013-04-20 14:10:42 +0000 @@ -3520,7 +3520,7 @@ { double sel, min_mp_pos, max_mp_pos; if (min_endp && !min_endp->key [0] ) + if (min_endp && (!min_endp->key [0] || !field->null_ptr)) { store_key_image_to_rec(field, (uchar *) min_endp->key, min_endp->length);

          The bug was fixed back then, in 10.0.2:

                  revno: 3427.18.37
                  revision-id: igor@askmonty.org-20130420091655-ir0aup5jdrl9skjz
                  parent: sanja@askmonty.org-20130418192204-g02z43zzxih2kbqn
                  committer: Igor Babaev <igor@askmonty.org>
                  branch nick: maria-10.0-mwl253-test
                  timestamp: Sat 2013-04-20 02:16:55 -0700
                  message:
                    Fixed bug mdev-4406.
                    This bug in the code of get_column_range_cardinality() could lead to
                    wrong estimates of number of records in ranges for non-nullable columns.  

          (Setting fix version to N/A because 10.0.2 is not on the list anymore).

          elenst Elena Stepanova added a comment - The bug was fixed back then, in 10.0.2: revno: 3427.18.37 revision-id: igor@askmonty.org-20130420091655-ir0aup5jdrl9skjz parent: sanja@askmonty.org-20130418192204-g02z43zzxih2kbqn committer: Igor Babaev <igor@askmonty.org> branch nick: maria-10.0-mwl253-test timestamp: Sat 2013-04-20 02:16:55 -0700 message: Fixed bug mdev-4406. This bug in the code of get_column_range_cardinality() could lead to wrong estimates of number of records in ranges for non-nullable columns. (Setting fix version to N/A because 10.0.2 is not on the list anymore).

          People

            Unassigned Unassigned
            igor Igor Babaev
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            Dates

              Created:
              Updated:
              Resolved:

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