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| # 压缩方式 |
| |
| 当时间序列写入并按照指定的类型编码为二进制数据后，IoTDB 会使用压缩技术对该数据进行压缩，进一步提升空间存储效率。虽然编码和压缩都旨在提升存储效率，但编码技术通常只适合特定的数据类型（如二阶差分编码只适合与 INT32 或者 INT64 编码，存储浮点数需要先将他们乘以 10m 以转换为整数），然后将它们转换为二进制流。压缩方式（SNAPPY）针对二进制流进行压缩，因此压缩方式的使用不再受数据类型的限制。 |
| |
| ## 基本压缩方式 |
| |
| IoTDB 允许在创建一个时间序列的时候指定该列的压缩方式。现阶段 IoTDB 支持以下几种压缩方式： |
| |
| * UNCOMPRESSED（不压缩） |
| * SNAPPY 压缩 |
| * LZ4 压缩 |
| * GZIP 压缩 |
| |
| 压缩方式的指定语法详见本文 [SQL 参考文档](../Reference/SQL-Reference.md)。 |
| |
| ## 压缩比统计信息 |
| |
| 压缩比统计信息文件：data/system/compression_ratio/Ratio-{ratio_sum}-{memtable_flush_time} |
| |
| * ratio_sum: memtable压缩比的总和 |
| * memtable_flush_time: memtable刷盘的总次数 |
| |
| 通过 `ratio_sum / memtable_flush_time` 可以计算出平均压缩比 |
