Details
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Bug
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Status: Closed (View Workflow)
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Major
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Resolution: Fixed
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10.0.9
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None
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)
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create table ten(a int);
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insert into ten values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
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create table t1(a int);
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insert into t1 select A.a from ten A, ten B, ten C;
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set histogram_size=20;
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set histogram_type='single_prec_hb';
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analyze table t1 persistent for all;
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set use_stat_tables='preferably';
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set optimizer_use_condition_selectivity=4;
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mysql> explain extended select * from t1 where a=2;
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+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
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+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
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| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 1000 | 8.67 | Using where |
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+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
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Not a precise estimate, but it is probably ok.
mysql> explain extended select * from t1 where a=1;
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+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
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+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
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| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 1000 | 8.67 | Using where |
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+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
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OK.
mysql> explain extended select * from t1 where a=0;
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+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
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+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
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| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 1000 | 100.00 | Using where |
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+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
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Boom. The number of rows with a=0 is 10% of the table. Not ok.
mysql> explain extended select * from t1 where a=-1;
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+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
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+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
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| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 1000 | 100.00 | Using where |
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+------+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
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-1 is not present at all. Not ok.
Attachments
Issue Links
- relates to
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- Closed
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