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今天同事讨论关于主键使用 varchar 和 int 的区别。
我现在总结的 3 个问题：
1、tablespace 中空间浪费
当然我们知道使用 varchar 可能会导致辅助索引比较大，因为用到 varchar 可能存储的字符较多，同时
在行头也存在一个可变字段字符区域(1-2) 字节
而辅助索引叶子结点毕竟都存储了主键值，这样至少会多 varchar 数据字节数量 +1(或者 2) 字节 - 4(int) 字节空间。
如果辅助索引比较多空间浪费是可想而知的。
2、辅助索引 B + 树扫描性能
由于辅助索引 B + 树的空间要求更大，虽然在 B + 树层次一般都是 3 层 - 4 层，索引单值定位 I / O 消耗并不明显，如果涉及到
范围查询(比如 PAGE_CUR_G), 需要访问的块就更多，同时比如例如辅助索引的 using index，需要访问的块自然
更多
3、比较更加复杂
innodb 在进行元组比较的时候，不管是 DML，select 都会涉及到元组的比较，同时回表的时候也涉及
到比较操作。而 varchar 类型的比较比 int 类型更为复杂一些。
那么我们就来分析第三个问题，第一个问题和第二个问题是显而易见的。
我这里数据库字符集为 latin1\latin1_swedish_ci
其实在 innodb 底层进行比较的时候都调用 cmp_data 这个函数
在 innodb 中有自己的定义的数据类型如下：
/*——————————————-*/
/* The ‘MAIN TYPE’ of a column */
#define DATA_MISSING 0 /* missing column */
#define DATA_VARCHAR 1 /* character varying of the
latin1_swedish_ci charset-collation; note
that the MySQL format for this, DATA_BINARY,
DATA_VARMYSQL, is also affected by whether the
‘precise type’ contains
DATA_MYSQL_TRUE_VARCHAR */
#define DATA_CHAR 2 /* fixed length character of the
latin1_swedish_ci charset-collation */
#define DATA_FIXBINARY 3 /* binary string of fixed length */
#define DATA_BINARY 4 /* binary string */
#define DATA_BLOB 5 /* binary large object, or a TEXT type;
if prtype & DATA_BINARY_TYPE == 0, then this is
actually a TEXT column (or a BLOB created
with < 4.0.14; since column prefix indexes
came only in 4.0.14, the missing flag in BLOBs
created before that does not cause any harm) */
#define DATA_INT 6 /* integer: can be any size 1 – 8 bytes */
#define DATA_SYS_CHILD 7 /* address of the child page in node pointer */
#define DATA_SYS 8 /* system column */
我们熟悉的 int 类型属于 DATA_INT 而 varchar 属于 DATA_VARCHAR，rowid 属于 DATA_SYS
在函数 cmp_data 根据各种类型的不同进行了不同比较的方式，这里就将 int 和 varchar
判断的方式进行说明：
1、innodb int 类型比较
实际上是在 cmp_data 中进行了大概的方式如下
if (len) {
#if defined __i386__ || defined __x86_64__ || defined _M_IX86 || defined _M_X64
/* Compare the first bytes with a loop to avoid the call
overhead of memcmp(). On x86 and x86-64, the GCC built-in
(repz cmpsb) seems to be very slow, so we will be calling the
libc version. http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43052
tracks the slowness of the GCC built-in memcmp().
We compare up to the first 4..7 bytes with the loop.
The (len & 3) is used for “normalizing” or
“quantizing” the len parameter for the memcmp() call,
in case the whole prefix is equal. On x86 and x86-64,
the GNU libc memcmp() of equal strings is faster with
len=4 than with len=3.
On other architectures than the IA32 or AMD64, there could
be a built-in memcmp() that is faster than the loop.
We only use the loop where we know that it can improve
the performance. */
for (ulint i = 4 + (len & 3); i > 0; i–) {
cmp = int(*data1++) – int(*data2++);
if (cmp) {
return(cmp);
}
if (!–len) {
break;
}
}
my_strnncollsp_simple
if (len) {
#endif /* IA32 or AMD64 */
cmp = memcmp(data1, data2, len);
if (cmp) {
return(cmp);
}
data1 += len;
data2 += len;
#if defined __i386__ || defined __x86_64__ || defined _M_IX86 || defined _M_X64
}
#endif /* IA32 or AMD64 */
}
cmp = (int) (len1 – len2);
if (!cmp || pad == ULINT_UNDEFINED) {
return(cmp);
}
可以看到整个方式比较简洁，对于我们常用的 x86_64 模型并没有直接使用 memcpy 进行而是
进行了优化在注释中也有说明，才出现了 for (ulint i = 4 + (len & 3); i > 0; i–)
部分，如果是 IA32 or AMD64 则直接使用 memcpy 进行比较。感兴趣的可以仔细阅读一下
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’)
at /root/mysql5.7.14/percona-server-5.7.14-7/strings/ctype-simple.c:165
#1 0x0000000001ab8ec2 in cmp_whole_field (mtype=1, prtype=524303, a=0x7fff57a71f93 “gaopeng”, a_length=7, b=0x7fffbd7e807f “gaopeng”, b_length=7)
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/rem/rem0cmp.cc:374
#2 0x0000000001aba827 in cmp_data (mtype=1, prtype=524303, data1=0x7fff57a71f93 “gaopeng”, len1=7, data2=0x7fffbd7e807f “gaopeng”, len2=7)
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/rem/rem0cmp.cc:468
#3 0x0000000001ab9a05 in cmp_dtuple_rec_with_match_bytes (dtuple=0x7fff48ed3280, rec=0x7fffbd7e807f “gaopeng”, index=0x7fff48ec78a0, offsets=0x7fff57a6bc50,
matched_fields=0x7fff57a6bf80, matched_bytes=0x7fff57a6bf78) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/rem/rem0cmp.cc:880
#4 0x0000000001a87fe2 in page_cur_search_with_match_bytes (block=0x7fffbcceafc0, index=0x7fff48ec78a0, tuple=0x7fff48ed3280, mode=PAGE_CUR_GE,
iup_matched_fields=0x7fff57a6cdf8, iup_matched_bytes=0x7fff57a6cdf0, ilow_matched_fields=0x7fff57a6cde8, ilow_matched_bytes=0x7fff57a6cde0, cursor=0x7fff57a713f8)
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/page/page0cur.cc:850
#5 0x0000000001c17a3e in btr_cur_search_to_nth_level (index=0x7fff48ec78a0, level=0, tuple=0x7fff48ed3280, mode=PAGE_CUR_GE, latch_mode=1, cursor=0x7fff57a713f0,
has_search_latch=0, file=0x2336938 “/root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/btr/btr0cur.cc”, line=5744, mtr=0x7fff57a70ee0)
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/btr/btr0cur.cc:1478
#6 0x0000000001c222bf in btr_estimate_n_rows_in_range_low (index=0x7fff48ec78a0, tuple1=0x7fff48ed3280, mode1=PAGE_CUR_GE, tuple2=0x7fff48ed32e0, mode2=PAGE_CUR_G,
nth_attempt=1) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/btr/btr0cur.cc:5744
#7 0x0000000001c22a09 in btr_estimate_n_rows_in_range (index=0x7fff48ec78a0, tuple1=0x7fff48ed3280, mode1=PAGE_CUR_GE, tuple2=0x7fff48ed32e0, mode2=PAGE_CUR_G)
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/btr/btr0cur.cc:6044
#8 0x00000000019b3e0e in ha_innobase::records_in_range (this=0x7fff48e7e3b0, keynr=1, min_key=0x7fff57a71680, max_key=0x7fff57a716a0)
at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/handler/ha_innodb.cc:13938
#9 0x0000000000f6ed5b in handler::multi_range_read_info_const (this=0x7fff48e7e3b0, keyno=1, seq=0x7fff57a71b90, seq_init_param=0x7fff57a71850, n_ranges_arg=0,
bufsz=0x7fff57a71780, flags=0x7fff57a71784, cost=0x7fff57a71d10) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/handler.cc:6440
#10 0x0000000000f70662 in DsMrr_impl::dsmrr_info_const (this=0x7fff48e7e820, keyno=1, seq=0x7fff57a71b90, seq_init_param=0x7fff57a71850, n_ranges=0,
bufsz=0x7fff57a71d70, flags=0x7fff57a71d74, cost=0x7fff57a71d10) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/handler.cc:7112
#11 0x00000000019be22f in ha_innobase::multi_range_read_info_const (this=0x7fff48e7e3b0, keyno=1, seq=0x7fff57a71b90, seq_init_param=0x7fff57a71850, n_ranges=0,
bufsz=0x7fff57a71d70, flags=0x7fff57a71d74, cost=0x7fff57a71d10) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/handler/ha_innodb.cc:21351
#12 0x000000000178c9e4 in check_quick_select (param=0x7fff57a71e30, idx=0, index_only=false, tree=0x7fff48e700e0, update_tbl_stats=true, mrr_flags=0x7fff57a71d74,
bufsize=0x7fff57a71d70, cost=0x7fff57a71d10) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/opt_range.cc:10030
#13 0x0000000001783305 in get_key_scans_params (param=0x7fff57a71e30, tree=0x7fff48e70058, index_read_must_be_used=false, update_tbl_stats=true,
cost_est=0x7fff57a74190) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/opt_range.cc:5812
#14 0x000000000177ce43 in test_quick_select (thd=0x7fff4801f4d0, keys_to_use=…, prev_tables=0, limit=18446744073709551615, force_quick_range=false,
interesting_order=st_order::ORDER_NOT_RELEVANT, tab=0x7fff48eacf20, cond=0x7fff48eacd50, needed_reg=0x7fff48eacf60, quick=0x7fff57a744c8)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/opt_range.cc:3066
#15 0x000000000158b9bc in get_quick_record_count (thd=0x7fff4801f4d0, tab=0x7fff48eacf20, limit=18446744073709551615)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_optimizer.cc:5942
#16 0x000000000158b073 in JOIN::estimate_rowcount (this=0x7fff48eac980) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_optimizer.cc:5689
#17 0x00000000015893b5 in JOIN::make_join_plan (this=0x7fff48eac980) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_optimizer.cc:5046
#18 0x000000000157d9b7 in JOIN::optimize (this=0x7fff48eac980) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_optimizer.cc:387
#19 0x00000000015fab71 in st_select_lex::optimize (this=0x7fff48aa45c0, thd=0x7fff4801f4d0) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_select.cc:1009
#20 0x00000000015f9284 in handle_query (thd=0x7fff4801f4d0, lex=0x7fff48021ab0, result=0x7fff48aa5dc8, added_options=0, removed_options=0)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_select.cc:164
#21 0x00000000015ac159 in execute_sqlcom_select (thd=0x7fff4801f4d0, all_tables=0x7fff48aa54b8) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:5391
#22 0x00000000015a4774 in mysql_execute_command (thd=0x7fff4801f4d0, first_level=true) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:2889
#23 0x00000000015ad12a in mysql_parse (thd=0x7fff4801f4d0, parser_state=0x7fff57a76600) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:5836
#24 0x00000000015a0fe9 in dispatch_command (thd=0x7fff4801f4d0, com_data=0x7fff57a76d70, command=COM_QUERY)
at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:1447
#25 0x000000000159fe1a in do_command (thd=0x7fff4801f4d0) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/sql_parse.cc:1010
#26 0x00000000016e1d6c in handle_connection (arg=0x6320740) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_handler_per_thread.cc:312
—Type <return> to continue, or q <return> to quit—
#27 0x0000000001d723f4 in pfs_spawn_thread (arg=0x6320530) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/perfschema/pfs.cc:2188
#28 0x0000003ca62079d1 in start_thread () from /lib64/libpthread.so.0
#29 0x0000003ca5ee8b6d in clone () from /lib64/libc.so.6
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