System Variables page states that tidb_external_ts is GLOBAL while other pages state it can also be SESSION level

[benmeadowcroft]

Error Report

1. What is the URL/path of the document related to this issue?

https://docs.pingcap.com/tidb/stable/system-variables/#tidb_external_ts-new-in-v640

2. How would you like to improve it?

Currently the page states that

tidb_external_ts New in v6.4.0
Scope: GLOBAL

However elsewhere in the documentation it states that it can be set either globally or at the session level.

I would like this ambiguity to be resolved by either updating the system variables page to note that SESSION scope is also supported, or update the other pages to correct the misinformation.

The page that indicate this can be SESSION scoped states:

Besides, TiDB provides a way to specify an exact point in time by setting the tidb_external_ts system variable on session or global level.

• see https://docs.pingcap.com/tidb/stable/stale-read/#usages

[Oreoxmt]

@YangKeao Could you please help confirm the scope of the tidb_external_ts variable?

I've tested it on v8.5.1, and it appears to be GLOBAL:

mysql> SELECT VARIABLE_NAME, VARIABLE_SCOPE FROM INFORMATION_SCHEMA.VARIABLES_INFO WHERE VARIABLE_NAME = 'tidb_external_ts';
+------------------+----------------+
| VARIABLE_NAME    | VARIABLE_SCOPE |
+------------------+----------------+
| tidb_external_ts | GLOBAL         |
+------------------+----------------+
1 row in set (0.01 sec)

Related PR: pingcap/tidb#38276

[YangKeao]

@YangKeao Could you please help confirm the scope of the tidb_external_ts variable?

I've tested it on v8.5.1, and it appears to be GLOBAL:

mysql> SELECT VARIABLE_NAME, VARIABLE_SCOPE FROM INFORMATION_SCHEMA.VARIABLES_INFO WHERE VARIABLE_NAME = 'tidb_external_ts';
+------------------+----------------+
| VARIABLE_NAME | VARIABLE_SCOPE |
+------------------+----------------+
| tidb_external_ts | GLOBAL |
+------------------+----------------+
1 row in set (0.01 sec)

Related PR: pingcap/tidb#38276

It's GLOBAL.

It can be turned on/off in session or global scope because the variable tidb_enable_external_ts_read is SESSION|GLOBAL. If the tidb_enable_external_ts_read = 'OFF', even when the tidb_external_ts has a value, it will have no effect. See the document https://docs.pingcap.com/tidb/stable/tidb-external-ts/#read-historical-data-using-the-tidb_external_ts-variable

[Oreoxmt]

To make the description in Stale Read Usage clearer and avoid misunderstandings, I have submitted these PRs for an update:

• stale read: update the description of tidb_external_ts #20667
• stale read: update the description of tidb_external_ts docs-cn#20134

@YangKeao PTAL