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## 查询性能追踪

IoTDB 支持使用 `TRACING` 语句来启用/禁用对查询语句的性能追踪，默认处于禁用状态。用户可以使用性能追踪工具来分析某些查询中存在的潜在性能问题。性能分析的日志文件默认存储在 `./data/tracing` 目录下。

启用 Tracing：

`IoTDB> TRACING ON`

禁用 Tracing：

`IoTDB> TRACING OFF`

由于一个 IoTDB 查询时间主要取决于查询的时间序列数、涉及访问的 Tsfile 文件数、需要扫描的 chunk 总数以及平均每个 chunk 的大小（指该 chunk 中包含的数据点的个数）。因此，目前性能分析包括的内容如下：

- Start time
- Query statement
- Number of series paths
- Number of sequence files
- Statistics of each sequence file
- Number of unSequence files
- Statistics of each unSequence file
- Number of chunks
- Average size of chunks
- Total cost time

### 示例

例如执行 `select * from root`，则 tracing 日志文件的内容会包括以下内容：

```
Query Id: 2 - Start time: 2020-06-28 10:53:54.727
Query Id: 2 - Query Statement: select * from root
Query Id: 2 - Number of series paths: 3
Query Id: 2 - Number of sequence files: 1
Query Id: 2 - SeqFile_1603336100446-1-0.tsfile root.sg.d1[1, 10000]
Query Id: 2 - Number of unsequence files: 1
Query Id: 2 - UnSeqFile_1603354798303-2-0.tsfile root.sg.d1[9, 1000]
Query Id: 2 - Number of chunks: 3
Query Id: 2 - Average size of chunks: 4113
Query Id: 2 - Total cost time: 11ms
```

为了避免多个查询同时执行导致输出信息乱序，在每条输出信息前均增加了该次查询的 Query Id，用户可以使用 `grep "Query Id: 2" tracing.txt` 来提取某次查询的所有追踪信息。
