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op-guide: fix variable description #670

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Oct 16, 2018
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op-guide: fix variable description #670

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7 changes: 4 additions & 3 deletions op-guide/history-read.md
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Original file line number Diff line number Diff line change
Expand Up @@ -10,7 +10,8 @@ This document describes how TiDB reads data from the history versions, how TiDB

## Feature description

TiDB implements a feature to read history data using the standard SQL interface directly without special clients or drivers. By using this feature,
TiDB implements a feature to read history data using the standard SQL interface directly without special clients or drivers. By using this feature:

- Even when data is updated or removed, its history versions can be read using the SQL interface.
- Even if the table structure changes after the data is updated, TiDB can use the old structure to read the history data.

Expand Down Expand Up @@ -107,7 +108,7 @@ Pay special attention to the following two variables:
Query OK, 0 rows affected (0.00 sec)
```

> **Note:** You should use `@@` instead of `@` before `tidb_snapshot` because `@@` is used to denote the global variable while `@` is used to denote the session variable.
> **Note:** You should use `@@` instead of `@` before `tidb_snapshot` because `@@` is used to denote the system variable while `@` is used to denote the user variable.

**Result:** The read from the following statement is the data before the update operation, which is the history data.

Expand Down Expand Up @@ -142,4 +143,4 @@ Pay special attention to the following two variables:
3 rows in set (0.00 sec)
```

> **Note:** You should use `@@` instead of `@` before `tidb_snapshot` because `@@` is used to denote the global variable while `@` is used to denote the session variable.
> **Note:** You should use `@@` instead of `@` before `tidb_snapshot` because `@@` is used to denote the system variable while `@` is used to denote the user variable.