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#BITMAP

## description
### Syntax

`TO_BITMAP(expr)` : Convert TINYINT,SMALLINT,INT type column to Bitmap.

`BITMAP_UNION(expr)` : Calculate the union of Bitmap, return the serialized Bitmap value.

`BITMAP_COUNT(expr)` : Calculate the distinct value number of a Bitmap.

`BITMAP_UNION_INT(expr)` : Calculate the distinct value number of TINYINT,SMALLINT and INT type column. Same as COUNT(DISTINCT expr)

`BITMAP_EMPTY()`: Generate empty bitmap column for insert into or load data.

Notice：

	1. TO_BITMAP function only receives TINYINT,SMALLINT,INT.
	2. BITMAP_UNION only receives following types of parameter:
		- Column with BITMAP_UNION aggregate type in AGGREGATE KEY mode.
		- TO_BITMAP function.

## 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_UNION,BITMAP_UNION_INT,TO_BITMAP