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Fixed bug #25407.

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The bug could cause choosing a sub-optimal execution plan for 
a single-table query if a unique index with many null keys were 
defined for the table. 
It happened because the code of the check_quick_keys function 
made an assumption that any key may occur in an unique index 
only once. Yet this is not true for keys with nulls that may 
have multiple occurrences in the index.
This commit is contained in:
igor@olga.mysql.com
2007-01-31 19:31:36 -08:00
parent 8760182170
commit 710136217b
4 changed files with 173 additions and 3 deletions
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4
mysql-test/r/null_key.result
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@ -30,7 +30,7 @@ id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ref a,b a 5 const 3 Using where; Using index
explain select * from t1 where a is null and b=9 or a is null and b=7 limit 3;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ref a,b a 5 const 2 Using where; Using index
1 SIMPLE t1 range a,b a 9 NULL 3 Using where; Using index
explain select * from t1 where a > 1 and a < 3 limit 1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 range a a 5 NULL 1 Using where; Using index
@ -258,7 +258,7 @@ INSERT INTO t1 VALUES (1,NULL),(2,NULL),(3,1),(4,2),(5,NULL),(6,NULL),(7,3),(8,4
INSERT INTO t2 VALUES (1,NULL),(2,NULL),(3,1),(4,2),(5,NULL),(6,NULL),(7,3),(8,4),(9,NULL),(10,NULL);
explain select id from t1 where uniq_id is null;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ref idx1 idx1 5 const 1 Using where
1 SIMPLE t1 ref idx1 idx1 5 const 5 Using where
explain select id from t1 where uniq_id =1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 const idx1 idx1 5 const 1