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Create a dataset:
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 one_k(a int);
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insert into one_k select A.a + B.a* 10 + C.a * 100 from ten A, ten B, ten C;
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create table t1 (a int, b int);
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insert into t1 select NULL, a from one_k;
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set optimizer_use_condition_selectivity=4;
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SET use_stat_tables = PREFERABLY;
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set histogram_size=100;
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analyze table t1 persistent for all;
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Ok, so we've got EITS statistics for t1.A. t1.a has only NULL values. Let's see how selectivity estimates work
MariaDB [test]> explain extended select * from t1 A straight_join t1 B where A.a < 5;
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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 | A | ALL | NULL | NULL | NULL | NULL | 1000 | 0.00 | Using where |
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| 1 | SIMPLE | B | ALL | NULL | NULL | NULL | NULL | 1000 | 100.00 | Using join buffer (flat, BNL join) |
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+------+-------------+-------+------+---------------+------+---------+------+------+----------+------------------------------------+
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So far, good. Now, let's try an "IS NULL":
MariaDB [test]> explain extended select * from t1 A straight_join t1 B where A.a is null;
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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 | A | ALL | NULL | NULL | NULL | NULL | 1000 | 0.00 | Using where |
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| 1 | SIMPLE | B | ALL | NULL | NULL | NULL | NULL | 1000 | 100.00 | Using join buffer (flat, BNL join) |
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+------+-------------+-------+------+---------------+------+---------+------+------+----------+------------------------------------+
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We get filtered=0 even if it should have been 100.
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