mysql中fetch源码分析

  嵌入式sql采用游标编程，一般有几步曲

1、EXEC SQL PREPARE sSql FROM :sql ;

2、EXEC SQL DECLARE cSql CURSOR FOR sSql ;

3、EXEC SQL OPEN cSql ;

4、EXEC SQL FETCH cSql INTO ...

while(sqlca.sqlcode....){ //判断结束条件

EXEC SQL FETCH cSql INTO ...

}

  那mysql中fetch是如何实现的呢，看一下fetch的主要流程

  先从sql_parse.cc中dispatch_command

  case COM_STMT_FETCH:

  {

        mysql_stmt_fetch(thd, packet, packet_length);

        break;

  }

sql_prepare.cc

void mysql_stmt_fetch(THD *thd, char *packet, uint packet_length)

{

....

        if (!(stmt= find_prepared_statement(thd, stmt_id)))--先看执行了prepare没有

       {

              char llbuf[22];

              my_error(ER_UNKNOWN_STMT_HANDLER, MYF(0), sizeof(llbuf),

              llstr(stmt_id, llbuf), "mysql_stmt_fetch");

              DBUG_VOID_RETURN;

      }

.....

       cursor= stmt->cursor; --是否declare游标

       if (!cursor) 

      {

            my_error(ER_STMT_HAS_NO_OPEN_CURSOR, MYF(0), stmt_id);

            DBUG_VOID_RETURN;

      }

 

      cursor->fetch(num_rows); --执行具体的fetch操作

 

      if (!cursor->is_open())--检查游标是否还打开

      {

            stmt->close_cursor();

            thd->cursor= 0;

            reset_stmt_params(stmt);

       }

 

       thd->restore_backup_statement(stmt, &stmt_backup);

       thd->stmt_arena= thd;

 

       DBUG_VOID_RETURN;

   }

       cursor->fetch(num_rows)有好几种实现，找个简单的分析一下

sql_cursor.cc

void Materialized_cursor::fetch(ulong num_rows)

{

       THD *thd= table->in_use;

 

        int res= 0;

        result->begin_dataset();

        for (fetch_limit+= num_rows; fetch_count < fetch_limit; fetch_count++)

        {

              if ((res= table->file->rnd_next(table->record[0])))

                     break;

              /* Send data only if the read was successful. */

              result->send_data(item_list); //直接从table里面读取，从临时表中读取

       }

 

        switch (res) { //根据不同结果，回复客户端不同信息

        case 0:

                thd->server_status|= SERVER_STATUS_CURSOR_EXISTS;

                result->send_eof();

                thd->server_status&= ~SERVER_STATUS_CURSOR_EXISTS;

                break;

       case HA_ERR_END_OF_FILE:

                thd->server_status|= SERVER_STATUS_LAST_ROW_SENT;

                result->send_eof();

                thd->server_status&= ~SERVER_STATUS_LAST_ROW_SENT;

                close();

                break;

       default:

                table->file->print_error(res, MYF(0));

                close();

                break;

      }

}

上面Materialized_cursor::fetch不满足事务隔离级别-游标稳定性，有时间看看

Sensitive_cursor::fetch能否满足事务隔离级别的要求