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op-guide: add notes about @@ #547

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Jul 26, 2018
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op-guide: add notes about @@ #547

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op-guide: add notes about @@
CaitinChen Jul 24, 2018
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Update wording
CaitinChen Jul 24, 2018
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Update wording
CaitinChen Jul 24, 2018
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CaitinChen Jul 25, 2018
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7 changes: 6 additions & 1 deletion op-guide/history-read.md
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Expand Up @@ -105,6 +105,9 @@ Pay special attention to the following two variables:
mysql> set @@tidb_snapshot="2016-10-08 16:45:26";
Query OK, 0 rows affected (0.00 sec)
```

> **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.

```sql
Expand Down Expand Up @@ -136,4 +139,6 @@ Pay special attention to the following two variables:
| 3 |
+------+
3 rows in set (0.00 sec)
```
```

> **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.