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[Doc] Bitmap explanation is incorrect #43110

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[Doc] Bitmap explanation is incorrect #43110

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8 changes: 4 additions & 4 deletions docs/zh/table_design/indexes/Bitmap_index.md
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Expand Up @@ -11,13 +11,13 @@ Bitmap 索引是一种使用 bitmap 的特殊数据库索引。bitmap 即为一

Bitmap 索引能够提高指定列的查询效率。如果一个查询条件命中列，StarRocks 即可使用[前缀索引](./Prefix_index_sort_key.md)提高查询效率，快速返回查询结果。但是前缀索引的长度有限，如果想要提高一个非前缀索引列的查询效率，即可以为这一列创建 Bitmap 索引。

Bitmap 索引一般适用于高基数列，基于列构建的 Bitmap 索引的选择性高，并且使用 Bitmap 索引后能筛选出较少数据行的场景。
Bitmap 索引一般适用于低基数列，基于列构建的 Bitmap 索引的选择性高，并且使用 Bitmap 索引后能筛选出较少数据行的场景。
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Hi, the document is correct.
We want users to create a bitmap index for high cardinality columns.
Suppose creating a bitmap index for a sex column that just has male/female, half of the records will be read. In that case, the query performance can not be accelerated by the index.

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I looked at the 2.5 version documentation and Wikipedia, and it says bitmap is suitable for low cardinality. The example on your official website is also a low-base example.

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在 StarRocks 中使用 SSB 100G 测试数据集验证 Bitmap 索引加速查询的效果，测试结果如下：

- 只有为高基数列创建 Bitmap 索引，查询性能才会有比较明显的提升（在此测试数据集中，基数达到 100000 数量级可以看到比较明显的性能提升）。
- 高基数列可以是高基数的单列，也可以是高基数的多列组合。
- 为低基数列创建 Bitmap 索引，查询性能基本没有提升甚至会下降。
- 只有为低基数列创建 Bitmap 索引，查询性能才会有比较明显的提升（在此测试数据集中，基数达到 100000 数量级可以看到比较明显的性能提升）。
- 低基数列可以是低基数的单列，也可以是低基数的多列组合。
- 为高基数列创建 Bitmap 索引，查询性能基本没有提升甚至会下降。

## 优势

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