mysql-handler

HANDLER语法

HANDLER tbl_name OPEN [ [AS] alias]

HANDLER tbl_name READ index_name { = | <= | >= | < | > } (value1,value2,...)
    [ WHERE where_condition ] [LIMIT ... ]
HANDLER tbl_name READ index_name { FIRST | NEXT | PREV | LAST }
    [ WHERE where_condition ] [LIMIT ... ]
HANDLER tbl_name READ { FIRST | NEXT }
    [ WHERE where_condition ] [LIMIT ... ]

HANDLER tbl_name CLOSE

通过HANDLER tbl_name OPEN打开一张表，无返回结果，实际上我们在这里声明了一个名为tb1_name的句柄。
通过HANDLER tbl_name READ FIRST获取句柄的第一行，通过READ NEXT依次获取其它行。最后一行执行之后再执行NEXT会返回一个空的结果。
通过HANDLER tbl_name CLOSE来关闭打开的句柄。

通过索引去查看的话可以按照一定的顺序，获取表中的数据。
通过HANDLER tbl_name READ index_name FIRST，获取句柄第一行（索引最小的一行），NEXT获取下一行，PREV获取前一行，LAST获取最后一行（索引最大的一行）。

通过索引列指定一个值，可以指定从哪一行开始。
通过HANDLER tbl_name READ index_name = value，指定从哪一行开始，通过NEXT继续浏览。

如果我们不想浏览一个表的所有行，可以使用where和limit子句。

HANDLE语句提供了直接访问存储引擎的接口。对于innodb和myism表都是可用的。

实例分析

创建测试表及测试数据

create table handler_table(  
    c1 int,   
    c2 varchar(10),   
    c3 int(10) 
);  
insert into handler_table values(2, 'name2', 002);  
insert into handler_table values(5, 'name5', 005);  
insert into handler_table values(1, 'name1', 001);  
insert into handler_table values(4, 'name4', 004);  
insert into handler_table values(3, 'name3', 003);

不通过索引打开查看表

打开句柄：

1	mysql> handler handler_table open;

查看表数据：

mysql> handler handler_table read first;
+------+-------+------+
| c1   | c2    | c3   |
+------+-------+------+
|    2 | name2 |    2 |
+------+-------+------+
mysql> handler handler_table read next;
+------+-------+------+
| c1   | c2    | c3   |
+------+-------+------+
|    5 | name5 |    5 |
+------+-------+------+
mysql> handler handler_table read next;
+------+-------+------+
| c1   | c2    | c3   |
+------+-------+------+
|    1 | name1 |    1 |
+------+-------+------+
mysql> handler handler_table read next;
+------+-------+------+
| c1   | c2    | c3   |
+------+-------+------+
|    4 | name4 |    4 |
+------+-------+------+
mysql> handler handler_table read next;
+------+-------+------+
| c1   | c2    | c3   |
+------+-------+------+
|    3 | name3 |    3 |
+------+-------+------+
mysql> handler handler_table read next;
Empty set (0.00 sec)

关闭句柄：

1 2	mysql> handler handler_table close; Query OK, 0 rows affected (0.00 sec)

通过索引打开查看表（FIRST,NEXT,PREV,LAST）
通过索引查看的话，可以按照索引的升序，从小到大，查看表信息。

创建索引：

1	mysql> create index handler_index on handler_table(c1);

打开句柄：

1	mysql> handler handler_table open as p;

查看表数据：

mysql> handler p read handler_index first;
+------+-------+------+
| c1   | c2    | c3   |
+------+-------+------+
|    1 | name1 |    1 |
+------+-------+------+
mysql> handler p read handler_index next;
+------+-------+------+
| c1   | c2    | c3   |
+------+-------+------+
|    2 | name2 |    2 |
+------+-------+------+
mysql> handler p read handler_index next;
+------+-------+------+
| c1   | c2    | c3   |
+------+-------+------+
|    3 | name3 |    3 |
+------+-------+------+
mysql> handler p read handler_index next;
+------+-------+------+
| c1   | c2    | c3   |
+------+-------+------+
|    4 | name4 |    4 |
+------+-------+------+
mysql> handler p read handler_index next;
+------+-------+------+
| c1   | c2    | c3   |
+------+-------+------+
|    5 | name5 |    5 |
+------+-------+------+
mysql> handler p read handler_index prev;
+------+-------+------+
| c1   | c2    | c3   |
+------+-------+------+
|    4 | name4 |    4 |
+------+-------+------+
mysql> handler p read handler_index last;
+------+-------+------+
| c1   | c2    | c3   |
+------+-------+------+
|    5 | name5 |    5 |
+------+-------+------+

关闭句柄：

1	mysql> handler p close;

通过索引打开查看表（=,<=,>=,<,>）
从index为2的地方开始
打开句柄：

1	mysql> handler handler_table open as p;

查看表数据：

mysql> handler p read handler_index = (2);
+------+-------+------+
| c1   | c2    | c3   |
+------+-------+------+
|    2 | name2 |    2 |
+------+-------+------+
mysql> handler p read handler_index next;     
+------+-------+------+
| c1   | c2    | c3   |
+------+-------+------+
|    3 | name3 |    3 |
+------+-------+------+
mysql> handler p read handler_index next;
+------+-------+------+
| c1   | c2    | c3   |
+------+-------+------+
|    4 | name4 |    4 |
+------+-------+------+
mysql> handler p read handler_index next;
+------+-------+------+
| c1   | c2    | c3   |
+------+-------+------+
|    5 | name5 |    5 |
+------+-------+------+
mysql> handler p read handler_index last;
+------+-------+------+
| c1   | c2    | c3   |
+------+-------+------+
|    5 | name5 |    5 |
+------+-------+------+

关闭句柄：

1	mysql> handler p close;

附加：语法实例参考

handler handler_table open;
handler handler_table open as p;
handler handler_table read first;
handler handler_table read next;
handler handler_table read first limit 3;
handler handler_table read next limit 3,3;
handler handler_table read first where c1 > 2 limit 2;
handler handler_table read next where c1 >2 limit 1,2;

create index handler_index on handler_table(c1);
handler handler_table open;
handler handler_table read handler_index first;
handler handler_table read handler_index next limit 3;
handler handler_table read handler_index PREV limit 3,3;
handler handler_table read handler_index LAST where c1 > 2 limit 2;
handler handler_table read handler_index LAST where c1 > 2 limit 1,2;
handler handler_table read handler_index = (3);
handler handler_table read handler_index <= (3) limit 2;
handler handler_table read handler_index >= (3) limit 1,2;
handler handler_table read handler_index < (4)  where c1 > 0 limit 2;
handler handler_table read handler_index > (1)  where c1 < 6 limit 2,2;
handler handler_table close;
drop index handler_index on handler_table;

handler与select的比较

4.1 select语句一次返回所有相关行，handler每次返回一行。
4.2 HANDLER涉及的分析较少，比SELECT更快
4.3 没有优化程序或查询校验开销
4.4 在两个管理程序请求之间，不需要锁定表。

注意事项

5.1 如果一个应用停止了，所有仍然打开的句柄将自动停止。
5.2 handler不支持分区表。
5.3 执行TRUNCATE TABLE会关闭所有在该表上打开的handler。
5.4 handler打开一个表时不锁表，也不对表进行快照，所以当表中数据实时更新时，handler将会失去指针的位置。