INFOBRIGHT 引擎下的DISTINCT的内存爆：
mysql> explain SELECT count(DISTINCT wuuid) as vnum FROM visit_info WHERE (1=1) and usertype in (-1) and (begintime >= 1380556800 and begintime <= 1387209600);
+----+-------------+------------+------+---------------+------+---------+------+----------+-----------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+------+---------------+------+---------+------+----------+-----------------------------------+
| 1 | SIMPLE | visit_info | ALL | NULL | NULL | NULL | NULL | 37778749 | Using where with pushed condition |
+----+-------------+------------+------+---------------+------+---------+------+----------+-----------------------------------+
1 row in set (0.65 sec)
mysql> explain SELECT count(DISTINCT uuid) as vnum FROM visit_info WHERE (1=1) and usertype in (-1) and (begintime >= 1380556800 and begintime <= 1387209600);
+----+-------------+------------+------+---------------+------+---------+------+----------+-----------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+------+---------------+------+---------+------+----------+-----------------------------------+
| 1 | SIMPLE | visit_info | ALL | NULL | NULL | NULL | NULL | 37778749 | Using where with pushed condition |
+----+-------------+------------+------+---------------+------+---------+------+----------+-----------------------------------+
1 row in set (0.00 sec)
mysql> SELECT count(DISTINCT uuid) as vnum FROM visit_info WHERE (1=1) and usertype in (-1) and (begintime >= 1380556800 and begintime <= 1387209600);
ERROR 9 (HY000): Brighthouse out of resources error: Insufficient memory/disk space
mysql> SELECT count( uuid) as vnum FROM visit_info WHERE (1=1) and usertype in (-1) and (begintime >= 1380556800 and begintime <= 1387209600);
+----------+
| vnum |
+----------+
| 37196097 |
+----------+
1 row in set (2.82 sec)
Distinct实现原理：
在数据库的设计中，如何实现Distinct操作呢？一般有两种基本思路：排序（Sort）法，哈希（Hash）法。
排序法将表格中的数据全部按照distinct指定的列为key进行排序，然后逐行迭代，
每迭代出一行数据都与上一行数据根据key作对比，如果相同，则丢弃当前行继续迭代下一行，
如果不同则输出。排序法带来的一个副作用就是数据输出按照key有序。
哈希法将表格中的数据全部按照distinct指定的列值为key作为hash key进行分桶，key相同的行自然就被区分出来了。
排序法在具体实现中会遇到这么一些问题：
1. 数据集超出了内存限制，如何排序？
2. 如何实现可以尽可能减少数据拷贝？
3. 如果已经有了Sort运算符，如何实现代码重用。
问题：上面两种方法在内存占用上那个更省？
最后说个题外话，distinct跟groupby蛮像的，那么他们的区别又在哪里呢？ 简单地说，distinct是一种很弱的groupby。详细见网上转载的一篇文章：
参考博客：
http://blog.csdn.net/maray/article/details/7634543