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#BITMAP

## description
### Syntax

`TO_BITMAP(expr)` : 将TINYINT,SMALLINT和INT类型的列转为Bitmap

`BITMAP_UNION(expr)` : 计算两个Bitmap的并集，返回值是序列化后的Bitmap值

`BITMAP_COUNT(expr)` : 计算Bitmap中不同值的个数

`BITMAP_UNION_INT(expr)` : 计算TINYINT,SMALLINT和INT类型的列中不同值的个数，返回值和
COUNT(DISTINCT expr)相同

`BITMAP_EMPTY()`: 生成空Bitmap列，用于insert或导入的时填充默认值


注意：

	1. TO_BITMAP 函数输入的类型必须是TINYINT,SMALLINT,INT
	2. BITMAP_UNION函数的参数目前仅支持： 
		- 聚合模型中聚合类型为BITMAP_UNION的列
		- TO_BITMAP 函数

## example

```
CREATE TABLE `bitmap_udaf` (
  `id` int(11) NULL COMMENT "",
  `id2` int(11)
) ENGINE=OLAP
DUPLICATE KEY(`id`)
DISTRIBUTED BY HASH(`id`) BUCKETS 10;


mysql> select bitmap_count(bitmap_union(to_bitmap(id2))) from bitmap_udaf;
+----------------------------------------------+
| bitmap_count(bitmap_union(to_bitmap(`id2`))) |
+----------------------------------------------+
|                                            6 |
+----------------------------------------------+

mysql> select bitmap_union_int (id2) from bitmap_udaf;
+-------------------------+
| bitmap_union_int(`id2`) |
+-------------------------+
|                       6 |
+-------------------------+


CREATE TABLE `bitmap_test` (
  `id` int(11) NULL COMMENT "",
  `id2` bitmap bitmap_union NULL
) ENGINE=OLAP
AGGREGATE KEY(`id`)
DISTRIBUTED BY HASH(`id`) BUCKETS 10;


mysql> select bitmap_count(bitmap_union(id2)) from bitmap_test;
+-----------------------------------+
| bitmap_count(bitmap_union(`id2`)) |
+-----------------------------------+
|                                 8 |
+-----------------------------------+

```

## keyword

BITMAP,BITMAP_COUNT,BITMAP_EMPTY,BITMAP_UNION,BITMAP_UNION_INT,TO_BITMAP