Bug#30665: Inconsistent optimization of IGNORE INDEX FOR {ORDER BY|GROUP BY}

The optimizer takes different execution paths during EXPLAIN than SELECT,
this fix relates only to EXPLAIN, hence no behavior changes.
The test of sort keys for ORDER BY was prohibited from considering keys
that were mentioned in IGNORE KEYS FOR ORDER BY. This led to two 
inconsistencies: One was that IGNORE INDEX FOR GROUP BY and 
IGNORE INDEX FOR ORDER BY gave apparently different EXPLAINs; the latter 
erroneously claimed to do filesort. The second inconsistency 
is that the test of sort keys is called twice, finding a sort key the first
time but not the second time, leading to the mentioned filesort.

Fixed by making the test of sort keys consider all enabled 
keys on the table. This test rejects keys that are not covering, and for 
covering keys the hint should be ignored anyway. 


mysql-test/r/group_by.result:
  Bug#30665: Changed test result. The plan gets more efficient here. 
  The output is included in order to show that it is still correct.
mysql-test/r/order_by.result:
  Bug#30665: Test result
mysql-test/t/group_by.test:
  Bug#30665: Changed test case to show correctness of changed plan
mysql-test/t/order_by.test:
  Bug#30665: Test case
sql/sql_select.cc:
  Bug#30665: 
  - the fix: Give test_if_skip_sort_order all keys not the subset of non-disabled keys.
  - Added comment to test_if_skip_sort_order

1 files changed, 147 insertions, 1 deletions

diff --git a/mysql-test/r/group_by.result b/mysql-test/r/group_by.result
index 2faf7832aca..0b38cc2290e 100644
--- a/mysql-test/r/group_by.result
+++ b/mysql-test/r/group_by.result
@@ -1096,7 +1096,153 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	144	Using index; Using filesort
 EXPLAIN SELECT a FROM t1 IGNORE INDEX FOR ORDER BY (PRIMARY,i2) ORDER BY a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	144	Using index; Using filesort
+1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	144	Using index
+SELECT a FROM t1 IGNORE INDEX FOR ORDER BY (PRIMARY,i2) ORDER BY a;
+a
+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
+92
+93
+94
+95
+96
+97
+98
+99
+100
+101
+102
+103
+104
+105
+106
+107
+108
+109
+110
+111
+112
+113
+114
+115
+116
+117
+118
+119
+120
+121
+122
+123
+124
+125
+126
+127
+128
+129
+130
+131
+132
+133
+134
+135
+136
+137
+138
+139
+140
+141
+142
+143
+144
 EXPLAIN SELECT a FROM t1 IGNORE INDEX FOR ORDER BY (PRIMARY)
 IGNORE INDEX FOR GROUP BY (i2) GROUP BY a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra