Astralidea commented on a change in pull request #4652:
URL: https://github.com/apache/incubator-doris/pull/4652#discussion_r495473912
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File path: docs/zh-CN/sql-reference/sql-statements/Data Types/BITMAP.md
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@@ -0,0 +1,48 @@
+---
+{
+ "title": "BITMAP",
+ "language": "zh-CN"
+}
+---
+
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements. See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership. The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License. You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
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+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied. See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+# BITMAP
+## description
+ BITMAP
+ BITMAP不能作为key列使用，建表时配合聚合类型为BITMAP_UNION。
+ 用户不需要指定长度和默认值。长度根据数据的聚合程度系统内控制。
+ 并且BITMAP列只能通过配套的bitmap_union_count、bitmap_union、bitmap_hash等函数进行查询或使用。
+
+ 离线场景下使用BITMAP会影响导入速度，在数据量大的情况下查询速度会慢于HLL，并优于Count Distinct。
+ 实时场景下BITMAP也不能做到完全的精确去重，通常误差小于千分之一。
Review comment:
实时场景下的BITMAP，通常不会引入全局字典，因为当全局字典大了之后时延很大，实时的吞吐和时延都会明显下降。导致查询和更新全局字典的成本和时延很高，会极大降低实时导入的性能。
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