1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
|
create table t1 (a int primary key, b int, c int, d int, e int, key ba (b,a), key bda (b,d,a), key cba (c,b,a), key cb (c,b), key d (d)) engine=aria;
insert into t1 select seq,seq,seq,seq,seq from seq_1_to_10;
insert into t1 values(20,2,2,2,2),(21,3,4,5,6);
#
# Get different scan costs
#
explain select sum(e) as "table_scan" from t1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 12
Last_query_cost 5.500000
explain select sum(a) as "index scan" from t1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index NULL PRIMARY 4 NULL 12 Using index
Last_query_cost 3.202929
#
# Range scans should be used if we don't examine all rows in the table
#
explain select count(a) from t1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE NULL NULL NULL NULL NULL NULL NULL Select tables optimized away
Last_query_cost 0.000000
explain select count(*) from t1 where a > 0;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index PRIMARY PRIMARY 4 NULL 12 Using where; Using index
Last_query_cost 3.202929
explain select count(*) from t1 where a > 1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index PRIMARY PRIMARY 4 NULL 12 Using where; Using index
Last_query_cost 3.202929
explain select count(*) from t1 where a > 2;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 range PRIMARY PRIMARY 4 NULL 11 Using where; Using index
Last_query_cost 2.997685
#
# Shorter indexes are prefered over longer indexs
#
explain select sum(a+b) from t1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index NULL ba 9 NULL 12 Using index
Last_query_cost 3.204394
explain select count(*) from t1 where b between 5 and 10;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 range ba,bda ba 5 NULL 6 Using where; Using index
Last_query_cost 1.872197
explain select sum(b+c) from t1 where b between 5 and 6 and c between 5 and 6;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 range ba,bda,cba,cb cb 10 NULL 2 Using where; Using index
Last_query_cost 0.970781
# Cost of 'd' should be slightly smaller as key 'ba' is longer than 'd'
explain select count(*) from t1 where b > 6;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 range ba,bda ba 5 NULL 5 Using where; Using index
Last_query_cost 1.646831
explain select count(*) from t1 where d > 6;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 range d d 5 NULL 5 Using where; Using index
Last_query_cost 1.646343
#
# Check covering index usage
#
explain select a,b,c from t1 where a=b;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index NULL cba 14 NULL 12 Using where; Using index
Last_query_cost 3.205859
#
# Prefer ref keys over ranges
#
explain select count(*) from t1 where b=2;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ref ba,bda ba 5 const 2 Using index
Last_query_cost 0.950732
explain select count(*) from t1 where b=2 and c=2;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ref ba,bda,cba,cb cb 10 const,const 2 Using index
Last_query_cost 0.950781
explain select count(*) from t1 where b=3 and c between 3 and 4;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 range ba,bda,cba,cb cb 10 NULL 2 Using where; Using index
Last_query_cost 0.970781
#
# Prefer eq keys over ref keys
#
explain select a,b,e from t1 where a=10 or a=11;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 range PRIMARY PRIMARY 4 NULL 2 Using index condition
Last_query_cost 2.520488
explain select a,b,e from t1 where d=10 or d=11;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 range d d 5 NULL 2 Using index condition
Last_query_cost 2.520537
drop table t1;
|
