还珠格格之花城公主:mysql的last_insert_id()是不是可靠的?
来源:百度文库 编辑:九乡新闻网 时间:2024/05/02 03:14:46
Mysql的API有个很有意义的函数last_insert_id()。这个函数的作用是,针对auto_increment字段,返回给定的 数据库链接,
上一步 INSERT 查询中产生的 AUTO_INCREMENT 的 ID 号。如果没有指定 数据库链接,则使用上一个打开的连接。
很多人质疑last_insert_id()是否是可靠的,以前我也犹豫过。
事实上的结果是mysql_insert_id决不会取错。首先做个测试,
在mysql_query("insert.....);之后立刻sleep(1100),其间再做些其他的insert.
然后发现在mysql_insert_id取的值都不会和其他的冲突。
看了半天mysql的代码。mysql_insert_id是这么定义的
my_ulonglong STDCALL mysql_insert_id(MYSQL *mysql)
{
return mysql->;last_used_con->;insert_id;
}
MYSQL是个结构体,里面包括数据库链接和一些当前数据库链接的状态值,
其中在MYSQL结构体里面有insert_id,mysql_insert_id函数返回的就是结构体里面的这个值。
typedef struct st_mysql
{
NET net; /* Communication parameters */
gptr connector_fd; /* ConnectorFd for SSL */
char *host,*user,*passwd,*unix_socket,*server_version,*host_info,*info;
char *db;
struct charset_info_st *charset;
MYSQL_FIELD *fields;
MEM_ROOT field_alloc;
my_ulonglong affected_rows;
my_ulonglong insert_id; /* id if insert on table with NEXTNR */
my_ulonglong extra_info; /* Used by mysqlshow */
unsigned long thread_id; /* Id for connection in server */
unsigned long packet_length;
unsigned int port;
unsigned long client_flag,server_capabilities;
unsigned int protocol_version;
unsigned int field_count;
unsigned int server_status;
unsigned int server_language;
unsigned int warning_count;
struct st_mysql_options options;
enum mysql_status status;
my_bool free_me; /* If free in mysql_close */
my_bool reconnect; /* set to 1 if automatic reconnect */
/* session-wide random string */
char scramble[SCRAMBLE_LENGTH+1];
/*
Set if this is the original connection, not a master or a slave we have
added though mysql_rpl_probe() or mysql_set_master()/ mysql_add_slave()
*/
my_bool rpl_pivot;
/*
Pointers to the master, and the next slave connections, points to
itself if lone connection.
*/
struct st_mysql* master, *next_slave;
struct st_mysql* last_used_slave; /* needed for round-robin slave pick */
/* needed for send/read/store/use result to work correctly with replication */
struct st_mysql* last_used_con;
LIST *stmts; /* list of all statements */
const struct st_mysql_methods *methods;
void *thd;
/*
Points to boolean flag in MYSQL_RES or MYSQL_STMT. We set this flag
from mysql_stmt_close if close had to cancel result set of this object.
*/
my_bool *unbuffered_fetch_owner;
} MYSQL;
把insert_id理解成max(id)是错误的,因为有并发存在。
结构体里面有insert_id是针对当前MYSQL连接的,而每次mysql_query操作在mysql服务器上可以理解为一次“原子”操作。