PostgreSQL和MySQL like区别

前言：今天在PostgreSQL中使用like，字段类型是int，执行语句报错，

1.表结构：都是用sysbench工具产生的

postgres=# d sbtest1;
                             Table "public.sbtest1"
 Column |      Type      |                      Modifiers                       
--------+----------------+------------------------------------------------------
 id     | integer        | not null default nextval('sbtest1_id_seq'::regclass)
 k      | integer        | not null default 0
 c      | character(120) | not null default ''::bpchar
 pad    | character(60)  | not null default ''::bpchar
Indexes:
    "sbtest1_pkey" PRIMARY KEY, btree (id)
    "k_1" btree (k)
MySQL

+-------+------------------+------+-----+---------+----------------+
| Field | Type             | Null | Key | Default | Extra          |
+-------+------------------+------+-----+---------+----------------+
| id    | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| k     | int(10) unsigned | NO   | MUL | 0       |                |
| c     | char(120)        | NO   |     |         |                |
| pad   | char(60)         | NO   |     |         |                |
+-------+------------------+------+-----+---------+----------------+
4 rows in set (0.03 sec)
2.在PostgreSQL里面执行语句

postgres=# select k from sbtest1 where k like '3%' limit 10;
ERROR:  operator does not exist: integer ~~ unknown
LINE 1: select k from sbtest1 where k like '3%' limit 10;
                                      ^
HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.

那么我们转换一下类型试一下，果然是可以了，那么意味着有索引页无法使用了

postgres=# select k from sbtest1 where k::text like '3%' limit 10;
   k   
-------
 39162
 39075
 37436
 36750
 34065
 37606
 38584
 34175
 35533
 38342
(10 rows)

果断全表扫描
postgres=# explain select k from sbtest1 where k::text like '3%' limit 10;
                            QUERY PLAN                             
-------------------------------------------------------------------
 Limit  (cost=0.00..63.64 rows=10 width=4)
   ->  Seq Scan on sbtest1  (cost=0.00..9577.31 rows=1505 width=4)
         Filter: ((k)::text ~~ '3%'::text)
(3 rows)

3.那么来看看MySQL是怎么样子滴，
不会报错，可以执行，是不是MySQL自动转换类型了，来个执行计划就懂了
(twDB)root@localhost [sysbench]> select k from sbtest1 where k like '3%' limit 10;
+------+
| k    |
+------+
|  319 |
| 3002 |
| 3073 |
| 3130 |
| 3173 |
| 3181 |
| 3211 |
| 3274 |
| 3279 |
| 3385 |
+------+
10 rows in set (0.00 sec)

哈哈，果然使用索引
(twDB)root@localhost [sysbench]> explain select k from sbtest1 where k like '3%' limit 10;
+----+-------------+---------+-------+---------------+------+---------+------+--------+--------------------------+
| id | select_type | table   | type  | possible_keys | key  | key_len | ref  | rows   | Extra                    |
+----+-------------+---------+-------+---------------+------+---------+------+--------+--------------------------+
|  1 | SIMPLE      | sbtest1 | index | k_1           | k_1  | 4       | NULL | 236197 | Using where; Using index |
+----+-------------+---------+-------+---------------+------+---------+------+--------+--------------------------+
1 row in set (0.03 sec)

(twDB)root@localhost [sysbench]>

结论
在PostgreSQL中，千万别对int类型进行like会无法使用索引的