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  1. MariaDB Server
  2. MDEV-5926

EITS: Histogram estimates for column=least_possible_value are wrong

        Details

        • Type: Bug
        • Status: Closed
        • Priority: Major
        • Resolution: Fixed
        • Affects Version/s: 10.0.9
        • Fix Version/s: 10.0.10
        • Component/s: None
        • Labels:

          Description

          This is based on observations over MDEV-4362 and MDEV-4364.

          Populate the table with 100 rows with a=0, 100 rows with a=1, etc. (the data distribution is perfectly uniform)

           
create table ten(a int);
insert into ten values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
create table t1(a int);
insert into t1 select A.a from ten A, ten B, ten C;
set histogram_size=20;
set histogram_type='single_prec_hb';
analyze table t1 persistent for all;
set use_stat_tables='preferably';
set optimizer_use_condition_selectivity=4;

          mysql> explain extended select * from t1 where a=2;
+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
|    1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL | 1000 |     8.67 | Using where |
+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+

          Not a precise estimate, but it is probably ok.

          mysql> explain extended select * from t1 where a=1;
+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
|    1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL | 1000 |     8.67 | Using where |
+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+

          OK.

          mysql> explain extended select * from t1 where a=0;
+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
|    1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL | 1000 |   100.00 | Using where |
+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+

          Boom. The number of rows with a=0 is 10% of the table. Not ok.

          mysql> explain extended select * from t1 where a=-1;
+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
|    1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL | 1000 |   100.00 | Using where |
+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+

          -1 is not present at all. Not ok.

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                relates to

                Bug - A problem which impairs or prevents the functions of the product. MDEV-5950 EITS: bad estimate for very skewed distributions

                • Major - Major loss of function.
                • Closed

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                  psergey Sergei Petrunia added a comment -

                  Committed a patch.

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                  psergey Sergei Petrunia added a comment - Committed a patch.
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                  psergey Sergei Petrunia added a comment -

                  Second variant of the patch pushed with review feedback addressed.

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                  psergey Sergei Petrunia added a comment - Second variant of the patch pushed with review feedback addressed.

                    People

                    • Assignee:
                      psergey Sergei Petrunia
                      Reporter:
                      psergey Sergei Petrunia
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                      • Created:
                        Updated:
                        Resolved: