mysql 层_MySQL：MySQL层比较函数调用

---

###一、问题来源

最近遇到一个日期比较的问题如下：

```

root@localhost:test:10:25:48>select from_unixtime(1596680255);

+---------------------------+

| from_unixtime(1596680255) |

+---------------------------+

| 2020-08-06 10:17:35 |

+---------------------------+

1 row in set (0.00 sec)

root@localhost:test:10:25:52>select from_unixtime(1596680255,'%Y-%m-%d') < '2020-08-1';

+----------------------------------------------------+

| from_unixtime(1596680255,'%Y-%m-%d') < '2020-08-1' |

+----------------------------------------------------+

| 1 |

+----------------------------------------------------+

1 row in set (2.70 sec)

root@localhost:test:10:25:59>select from_unixtime(1596680255) < '2020-08-1';

+-----------------------------------------+

| from_unixtime(1596680255) < '2020-08-1' |

+-----------------------------------------+

| 0 |

+-----------------------------------------+

1 row in set (2.36 sec)

```

按理来说from_unixtime(1596680255,'%Y-%m-%d') < '2020-08-1' 应该是false才对，但是返回为true，因此怀疑为字符串比较的方式

###二、问题验证

首先关闭gtid通过create table as 来验证一下字段类型如下：

```

create table testit951

as

select from_unixtime(1596678161,'%Y-%m-%d') as "tt"

create table testit952

as

select from_unixtime(1596678161) as "tt"

```

通过这种方式发现两种建表字段为varchar和datetime类型。通过源码验证可以看到如下：

```

Breakpoint 5, Arg_comparator::compare_string (this=0x7fffe40167c0) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/item_cmpfunc.cc:1672

1672 if ((res1= (*a)->val_str(&value1)))

(gdb) c

Continuing.

Breakpoint 4, Arg_comparator::compare_datetime (this=0x7fffe40164c8) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/item_cmpfunc.cc:1509

1509 THD *thd= current_thd;

(gdb) c

Continuing.

```

可以看到内部通过了string和datetime两种类型来比较。因此得到证明。最后留下比较函数的位置：

Item_cmpfunc.cc


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