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op-guide, sql: add description about TSO support #492

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Jun 11, 2018
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op-guide, sql: add description about TSO support #492

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op-guide, sql: add description about TSO support
lilin90 Jun 11, 2018
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op-guide: address the comment
lilin90 Jun 11, 2018
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2 changes: 1 addition & 1 deletion op-guide/history-read.md
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Expand Up @@ -20,7 +20,7 @@ The `tidb_snapshot` system variable is introduced to support reading history dat
- The variable is valid in the `Session` scope.
- Its value can be modified using the `Set` statement.
- The data type for the variable is text.
- The variable is to record time in the following format: “2016-10-08 16:45:26.999”. Generally, the time can be set to seconds like in “2016-10-08 16:45:26”.
- The variable can store TSO (Timestamp Oracle) and time. TSO is a globally unique time service, which obtains information from PD. The time that this variable records uses the following format: "2016-10-08 16:45:26.999". Generally, the time can be set to just seconds like in "2016-10-08 16:45:26".
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@zhexuany zhexuany Jun 11, 2018 •
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The variable accepts TSO (Timestamp Oracle) and datetime. TSO is a globally unique time service, which is obtained from PD. The acceptable datetime format is "2016-10-08 16:45:26.999". Generally, the datetime can be set using second precision, e.g, "2016-10-08 16:45:26".

Above translation resolves the confusion. @lilin90 PTAL

- When the variable is set, TiDB creates a Snapshot using its value as the timestamp, just for the data structure and there is no any overhead. After that, all the `Select` operations will read data from this Snapshot.

> **Note:** Because the timestamp in TiDB transactions is allocated by Placement Driver (PD), the version of the stored data is also marked based on the timestamp allocated by PD. When a Snapshot is created, the version number is based on the value of the `tidb_snapshot` variable. If there is a large difference between the local time of the TiDB server and the PD server, use the time of the PD server.
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2 changes: 1 addition & 1 deletion sql/tidb-specific.md
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Expand Up @@ -20,7 +20,7 @@ If you need to set the global variable, run:

- Scope: SESSION
- Default value: ""
- This variable is used to set the time point at which the data is read by the session. For example, when you set the variable to "2017-11-11 20:20:20", the current session reads the data of this moment.
- This variable is used to set the time point at which the data is read by the session. For example, when you set the variable to "2017-11-11 20:20:20" or a TSO number like "400036290571534337", the current session reads the data of this moment.

### tidb_import_data

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