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MWL#89
Fixed query plans with loose index scan degraded into index scan. Analysis: With MWL#89 subqueries are no longer executed and substituted during the optimization of the outer query. As a result subquery predicates that were previously executed and substituted by a constant before the range optimizer were present as regular subquery predicates during range optimization. The procedure check_group_min_max_predicates() had a naive test that ruled out all queries with subqueries in the WHERE clause. This resulted in worse plans with MWL#89. Solution: The solution is to refine the test in check_group_min_max_predicates() to check if each MIN/MAX argument is referred to by a subquery predicate.
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3 changed files with 222 additions and 16 deletions
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@ -1360,12 +1360,158 @@ group by a1,a2,b;
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t1 index NULL idx_t1_1 163 NULL 128 Using where; Using index
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2 DEPENDENT SUBQUERY t2 index NULL idx_t2_1 163 NULL 164 Using where; Using index
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select a1,a2,b,min(c),max(c) from t1
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where exists ( select * from t2 where t2.c = t1.c )
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group by a1,a2,b;
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a1 a2 b min(c) max(c)
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a a a a111 d111
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a a b e112 h112
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a b a i121 l121
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a b b m122 p122
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b a a a211 d211
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b a b e212 h212
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b b a i221 l221
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b b b m222 p222
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c a a a311 d311
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c a b e312 h312
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c b a i321 l321
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c b b m322 p322
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d a a a411 d411
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d a b e412 h412
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d b a i421 l421
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d b b m422 p422
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explain select a1,a2,b,min(c),max(c) from t1
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where exists ( select * from t2 where t2.c > 'b1' )
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group by a1,a2,b;
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t1 range NULL idx_t1_1 147 NULL 17 Using index for group-by
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1 PRIMARY t1 range NULL idx_t1_1 147 NULL 17 Using where; Using index for group-by
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2 SUBQUERY t2 index NULL idx_t2_1 163 NULL 164 Using where; Using index
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select a1,a2,b,min(c),max(c) from t1
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where exists ( select * from t2 where t2.c > 'b1' )
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group by a1,a2,b;
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a1 a2 b min(c) max(c)
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a a a a111 d111
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a a b e112 h112
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a b a i121 l121
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a b b m122 p122
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b a a a211 d211
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b a b e212 h212
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b b a i221 l221
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b b b m222 p222
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c a a a311 d311
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c a b e312 h312
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c b a i321 l321
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c b b m322 p322
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d a a a411 d411
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d a b e412 h412
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d b a i421 l421
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d b b m422 p422
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explain select a1,a2,b,c,min(c), max(c) from t1
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where exists ( select * from t2 where t1.b > 'a' and t2.c > 'b1' )
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group by a1,a2,b;
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t1 range NULL idx_t1_1 147 NULL 17 Using where; Using index for group-by
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2 DEPENDENT SUBQUERY t2 index NULL idx_t2_1 163 NULL 164 Using where; Using index
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select a1,a2,b,c,min(c), max(c) from t1
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where exists ( select * from t2 where t1.b > 'a' and t2.c > 'b1' )
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group by a1,a2,b;
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a1 a2 b c min(c) max(c)
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a a b h112 e112 h112
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a b b p122 m122 p122
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b a b h212 e212 h212
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b b b p222 m222 p222
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c a b h312 e312 h312
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c b b p322 m322 p322
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d a b h412 e412 h412
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d b b p422 m422 p422
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explain select a1,a2,b,c,min(c), max(c) from t1
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where exists ( select * from t2
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where t2.c in (select c from t3 where t3.c > t1.b) and
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t2.c > 'b1' )
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group by a1,a2,b;
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t1 range NULL idx_t1_1 147 NULL 17 Using where; Using index for group-by
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2 DEPENDENT SUBQUERY t2 index NULL idx_t2_1 163 NULL 164 Using where; Using index
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2 DEPENDENT SUBQUERY t3 index NULL idx_t3_1 10 NULL 192 Using where; Using index; FirstMatch(t2)
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select a1,a2,b,c,min(c), max(c) from t1
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where exists ( select * from t2
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where t2.c in (select c from t3 where t3.c > t1.b) and
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t2.c > 'b1' )
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group by a1,a2,b;
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a1 a2 b c min(c) max(c)
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a a a d111 a111 d111
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a a b h112 e112 h112
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a b a l121 i121 l121
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a b b p122 m122 p122
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b a a d211 a211 d211
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b a b h212 e212 h212
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b b a l221 i221 l221
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b b b p222 m222 p222
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c a a d311 a311 d311
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c a b h312 e312 h312
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c b a l321 i321 l321
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c b b p322 m322 p322
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d a a d411 a411 d411
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d a b h412 e412 h412
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d b a l421 i421 l421
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d b b p422 m422 p422
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explain select a1,a2,b,c,min(c), max(c) from t1
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where exists ( select * from t2 where t1.c > 'a' and t2.c > 'b1' )
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group by a1,a2,b;
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t1 index NULL idx_t1_1 163 NULL 128 Using where; Using index
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2 DEPENDENT SUBQUERY t2 index NULL idx_t2_1 163 NULL 164 Using where; Using index
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select a1,a2,b,c,min(c), max(c) from t1
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where exists ( select * from t2 where t1.c > 'a' and t2.c > 'b1' )
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group by a1,a2,b;
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a1 a2 b c min(c) max(c)
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a a a a111 a111 d111
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a a b e112 e112 h112
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a b a i121 i121 l121
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a b b m122 m122 p122
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b a a a211 a211 d211
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b a b e212 e212 h212
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b b a i221 i221 l221
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b b b m222 m222 p222
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c a a a311 a311 d311
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c a b e312 e312 h312
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c b a i321 i321 l321
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c b b m322 m322 p322
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d a a a411 a411 d411
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d a b e412 e412 h412
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d b a i421 i421 l421
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d b b m422 m422 p422
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explain select a1,a2,b,c,min(c), max(c) from t1
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where exists ( select * from t2
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where t2.c in (select c from t3 where t3.c > t1.c) and
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t2.c > 'b1' )
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group by a1,a2,b;
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t1 index NULL idx_t1_1 163 NULL 128 Using where; Using index
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2 DEPENDENT SUBQUERY t2 index NULL idx_t2_1 163 NULL 164 Using where; Using index
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2 DEPENDENT SUBQUERY t3 index NULL idx_t3_1 10 NULL 192 Using where; Using index; FirstMatch(t2)
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select a1,a2,b,c,min(c), max(c) from t1
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where exists ( select * from t2
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where t2.c in (select c from t3 where t3.c > t1.c) and
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t2.c > 'b1' )
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group by a1,a2,b;
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a1 a2 b c min(c) max(c)
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a a a a111 a111 d111
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a a b e112 e112 h112
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a b a i121 i121 l121
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a b b m122 m122 p122
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b a a a211 a211 d211
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b a b e212 e212 h212
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b b a i221 i221 l221
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b b b m222 m222 p222
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c a a a311 a311 d311
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c a b e312 e312 h312
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c b a i321 i321 l321
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c b b m322 m322 o322
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d a a a411 a411 d411
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d a b e412 e412 h412
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d b a i421 i421 l421
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d b b m422 m422 o422
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explain select a1,a2,b,min(c),max(c) from t1 where (a1 >= 'c' or a2 < 'b') and (b > 'a') group by a1,a2,b;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t1 range idx_t1_0,idx_t1_1,idx_t1_2 idx_t1_1 147 NULL 17 Using where; Using index for group-by
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@ -2246,11 +2392,11 @@ EXPLAIN SELECT 1 FROM t1 AS t1_outer WHERE EXISTS
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(SELECT max(b) FROM t1 GROUP BY a HAVING a < 2);
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t1_outer index NULL a 10 NULL 15 Using index
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2 SUBQUERY t1 index NULL a 10 NULL 15 Using index
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2 SUBQUERY t1 index NULL a 10 NULL 1 Using index
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EXPLAIN SELECT 1 FROM t1 AS t1_outer WHERE
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(SELECT max(b) FROM t1 GROUP BY a HAVING a < 2) > 12;
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY NULL NULL NULL NULL NULL NULL NULL Impossible WHERE
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1 PRIMARY t1_outer index NULL a 10 NULL 15 Using index
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2 SUBQUERY t1 range NULL a 5 NULL 8 Using index for group-by
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EXPLAIN SELECT 1 FROM t1 AS t1_outer WHERE
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a IN (SELECT max(b) FROM t1 GROUP BY a HAVING a < 2);
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@ -406,11 +406,61 @@ explain select a1,a2,b,min(c),max(c) from t1
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where exists ( select * from t2 where t2.c = t1.c )
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group by a1,a2,b;
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select a1,a2,b,min(c),max(c) from t1
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where exists ( select * from t2 where t2.c = t1.c )
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group by a1,a2,b;
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# the sub-select is unrelated to MIN/MAX
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explain select a1,a2,b,min(c),max(c) from t1
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where exists ( select * from t2 where t2.c > 'b1' )
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group by a1,a2,b;
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select a1,a2,b,min(c),max(c) from t1
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where exists ( select * from t2 where t2.c > 'b1' )
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group by a1,a2,b;
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# correlated subselect that doesn't reference the min/max argument
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explain select a1,a2,b,c,min(c), max(c) from t1
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where exists ( select * from t2 where t1.b > 'a' and t2.c > 'b1' )
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group by a1,a2,b;
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select a1,a2,b,c,min(c), max(c) from t1
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where exists ( select * from t2 where t1.b > 'a' and t2.c > 'b1' )
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group by a1,a2,b;
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explain select a1,a2,b,c,min(c), max(c) from t1
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where exists ( select * from t2
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where t2.c in (select c from t3 where t3.c > t1.b) and
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t2.c > 'b1' )
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group by a1,a2,b;
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select a1,a2,b,c,min(c), max(c) from t1
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where exists ( select * from t2
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where t2.c in (select c from t3 where t3.c > t1.b) and
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t2.c > 'b1' )
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group by a1,a2,b;
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# correlated subselect that references the min/max argument
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explain select a1,a2,b,c,min(c), max(c) from t1
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where exists ( select * from t2 where t1.c > 'a' and t2.c > 'b1' )
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group by a1,a2,b;
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select a1,a2,b,c,min(c), max(c) from t1
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where exists ( select * from t2 where t1.c > 'a' and t2.c > 'b1' )
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group by a1,a2,b;
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explain select a1,a2,b,c,min(c), max(c) from t1
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where exists ( select * from t2
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where t2.c in (select c from t3 where t3.c > t1.c) and
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t2.c > 'b1' )
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group by a1,a2,b;
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select a1,a2,b,c,min(c), max(c) from t1
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where exists ( select * from t2
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where t2.c in (select c from t3 where t3.c > t1.c) and
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t2.c > 'b1' )
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group by a1,a2,b;
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# A,B,C) Predicates referencing mixed classes of attributes
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# plans
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@ -11102,7 +11102,7 @@ static bool get_constant_key_infix(KEY *index_info, SEL_ARG *index_range_tree,
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uchar *key_infix, uint *key_infix_len,
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KEY_PART_INFO **first_non_infix_part);
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static bool
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check_group_min_max_predicates(COND *cond, Item_field *min_max_arg_item,
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check_group_min_max_predicates(Item *cond, Item_field *min_max_arg_item,
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Field::imagetype image_type);
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static void
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@ -11676,13 +11676,16 @@ get_best_group_min_max(PARAM *param, SEL_TREE *tree)
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*/
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static bool
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check_group_min_max_predicates(COND *cond, Item_field *min_max_arg_item,
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check_group_min_max_predicates(Item *cond, Item_field *min_max_arg_item,
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Field::imagetype image_type)
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{
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DBUG_ENTER("check_group_min_max_predicates");
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DBUG_ASSERT(cond && min_max_arg_item);
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cond= cond->real_item();
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if (cond->get_cached_item())
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cond= cond->get_cached_item();
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Item::Type cond_type= cond->type();
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if (cond_type == Item::COND_ITEM) /* 'AND' or 'OR' */
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{
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@ -11699,18 +11702,25 @@ check_group_min_max_predicates(COND *cond, Item_field *min_max_arg_item,
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}
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/*
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TODO:
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This is a very crude fix to handle sub-selects in the WHERE clause
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(Item_subselect objects). With the test below we rule out from the
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optimization all queries with subselects in the WHERE clause. What has to
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be done, is that here we should analyze whether the subselect references
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the MIN/MAX argument field, and disallow the optimization only if this is
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so.
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Disallow loose index scan if the MIN/MAX argument field is referenced by
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a subquery in the WHERE clause.
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*/
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if (cond_type == Item::SUBSELECT_ITEM ||
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(cond->get_cached_item() &&
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cond->get_cached_item()->type() == Item::SUBSELECT_ITEM))
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DBUG_RETURN(FALSE);
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if (cond_type == Item::SUBSELECT_ITEM)
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{
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Item_subselect *subs_cond= (Item_subselect*) cond;
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if (subs_cond->is_correlated)
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{
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DBUG_ASSERT(subs_cond->depends_on.elements > 0);
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List_iterator_fast<Item*> li(subs_cond->depends_on);
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Item **dep;
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while ((dep= li++))
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{
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if ((*dep)->eq(min_max_arg_item, FALSE))
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DBUG_RETURN(FALSE);
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}
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}
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DBUG_RETURN(TRUE);
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}
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/*
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Condition of the form 'field' is equivalent to 'field <> 0' and thus
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