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sql-plan-replayer: add doc for historical stats integration into the plan replayer #14243

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Aug 9, 2023
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sql-plan-replayer: add doc for historical stats integration into the plan replayer #14243

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time-and-fate Jul 17, 2023
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8 changes: 7 additions & 1 deletion sql-plan-replayer.md
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Expand Up @@ -19,7 +19,7 @@ You can use `PLAN REPLAYER` to save the on-site information of a TiDB cluster. T
{{< copyable "sql" >}}

```sql
PLAN REPLAYER DUMP EXPLAIN [ANALYZE] sql-statement;
PLAN REPLAYER DUMP EXPLAIN [ANALYZE] [WITH STATS AS OF TIMESTAMP expression] sql-statement;
```

Based on `sql-statement`, TiDB sorts out and exports the following on-site information:
Expand All @@ -33,6 +33,10 @@ Based on `sql-statement`, TiDB sorts out and exports the following on-site infor
- The result of `EXPLAIN [ANALYZE] sql-statement`
- Some internal procudures of query optimization

If historical statistics are [enabled](/system-variables.md#tidb_enable_historical_stats), you can specify a time in the `PLAN REPLAYER` statement to get the historical statistics at the corresponding time. You can directly specify the time and date, or specify a timestamp here. When the time is specified, TiDB will look for the historical statistics before the specified time, and export the latest one among them.
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If there are no historical statistics that satisfy the specified time, TiDB will export the latest statistics, as if the time is not specified, and also print error messages in the `errors.txt` of the output `ZIP` file.
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> **Note:**
>
> `PLAN REPLAYER` **DOES NOT** export any table data.
Expand All @@ -48,6 +52,8 @@ insert into t values(1,1), (2, 2), (3, 3);
analyze table t;

plan replayer dump explain select * from t;
plan replayer dump with stats as of timestamp '2023-07-17 12:00:00' explain select * from t;
plan replayer dump with stats as of timestamp '442012134592479233' explain select * from t;
```

`PLAN REPLAYER DUMP` packages the table information above into a `ZIP` file and returns the file identifier as the execution result.
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