2、innodb varchar类型比较
实际上这个比较会通过cmp_data->cmp_whole_field->my_strnncollsp_simple调用最终调用
my_strnncollsp_simple完成，而比如order by 会调用my_strnxfrm_simple他们都在一个
文件中。
下面是整个my_strnncollsp_simple函数
/*
Compare strings, discarding end space
SYNOPSIS
my_strnncollsp_simple()
cs character set handler
a First string to compare
a_length Length of 'a'
b Second string to compare
b_length Length of 'b'
diff_if_only_endspace_difference
Set to 1 if the strings should be regarded as different
if they only difference in end space
IMPLEMENTATION
If one string is shorter as the other, then we space extend the other
so that the strings have equal length.
This will ensure that the following things hold:
"a" == "a "
"a\0" < "a"
"a\0" < "a "
RETURN
< 0 a < b
= 0 a == b
> 0 a > b
*/
int my_strnncollsp_simple(const CHARSET_INFO *cs, const uchar *a,
size_t a_length, const uchar *b, size_t b_length,
my_bool diff_if_only_endspace_difference)
{
const uchar *map= cs->sort_order, *end;
size_t length;
int res;
#ifndef VARCHAR_WITH_DIFF_ENDSPACE_ARE_DIFFERENT_FOR_UNIQUE
diff_if_only_endspace_difference= 0;
#endif
end= a + (length= MY_MIN(a_length, b_length));
while (a < end)
{
if (map[*a++] != map[*b++])
return ((int) map[a[-1]] - (int) map[b[-1]]);
}
res= 0;
if (a_length != b_length)
{
int swap= 1;
if (diff_if_only_endspace_difference)
res= 1; /* Assume 'a' is bigger */
/*
Check the next not space character of the longer key. If it's < ' ',
then it's smaller than the other key.
*/
if (a_length < b_length)
{
/* put shorter key in s */
a_length= b_length;
a= b;
swap= -1; /* swap sign of result */
res= -res;
}
for (end= a + a_length-length; a < end ; a++)
{
if (map[*a] != map[' '])
return (map[*a] < map[' ']) ? -swap : swap;
}
}
return res;
}
其中*map= cs->sort_order比较关键这是内存中已经存储好的字符集的顺序，
循环进行
map[*a++] != map[*b++]
*a++和*b++ 会得到的字符集编码，然后在整个排序好的字符数组中找，
则得到了实际字符集编码进行比较，不管是比较的复杂度还是需要比较的
长度 varchar很可能都远远大于int类型，下面是打印cs->sort_order这片
内存区域前128字节得到的结果，
(gdb) x/128bx 0x258b000
0x258b000 : 0x00 0x01 0x02 0x03 0x04 0x05 0x06 0x07
0x258b008 : 0x08 0x09 0x0a 0x0b 0x0c 0x0d 0x0e 0x0f
0x258b010 : 0x10 0x11 0x12 0x13 0x14 0x15 0x16 0x17
0x258b018 : 0x18 0x19 0x1a 0x1b 0x1c 0x1d 0x1e 0x1f
0x258b020 : 0x20 0x21 0x22 0x23 0x24 0x25 0x26 0x27
0x258b028 : 0x28 0x29 0x2a 0x2b 0x2c 0x2d 0x2e 0x2f
0x258b030 : 0x30 0x31 0x32 0x33 0x34 0x35 0x36 0x37
0x258b038 : 0x38 0x39 0x3a 0x3b 0x3c 0x3d 0x3e 0x3f
0x258b040 : 0x40 0x41 0x42 0x43 0x44 0x45 0x46 0x47
0x258b048 : 0x48 0x49 0x4a 0x4b 0x4c 0x4d 0x4e 0x4f
0x258b050 : 0x50 0x51 0x52 0x53 0x54 0x55 0x56 0x57
0x258b058 : 0x58 0x59 0x5a 0x5b 0x5c 0x5d 0x5e 0x5f
0x258b060 : 0x60 0x41 0x42 0x43 0x44 0x45 0x46 0x47
0x258b068 : 0x48 0x49 0x4a 0x4b 0x4c 0x4d 0x4e 0x4f
0x258b070 : 0x50 0x51 0x52 0x53 0x54 0x55 0x56 0x57
0x258b078 : 0x58 0x59 0x5a 0x7b 0x7c 0x7d 0x7e 0x7f
而从内存的地址0x258b000我们也能看到他确实是存在于堆内存空间中，它是一片堆内存区域。
下面是varchar比较的调用栈帧以备后用
#0 my_strnncollsp_simple (cs=0x2d4b9c0, a=0x7fff57a71f93 "gaopeng", a_length=7, b=0x7fffbd7e807f "gaopeng", b_length=7, diff_if_only_endspace_difference=0 '\000')