Add a note about potential caveats for the "session_timezone" setting

[alexey-milovidov]

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

The session_timezone setting is demoted to experimental.

[robot-ch-test-poll3]

This is an automated comment for commit 126ee1d with description of existing statuses. It's updated for the latest CI running
The full report is available here
The overall status of the commit is 🟡 pending

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[den-crane]

It's really important feature. All other databases has it.
I don't understand how users use Clickhouse with BI systems without it.

Right now all my SQL queries stuffed with macro {ui_timezone}
Some SQL queries have 20 timezone macro substitution.

  tuple(nullIf(toDate(toTimeZone(utc_access_hour, 'US/Eastern')), toDate(0)))  as `ctuple_null_135`
...
      (utc_access_hour <= toDateTime('2023-07-22 23:00:00', 'US/Eastern'))
  AND  (utc_access_hour >= toDateTime('2023-07-22 00:00:00', 'US/Eastern'))

I was going to replace this boilerplate with one pass session_timezone to the SQL session.

[zvonand]

I will take a deeper look into potential problems that may occur.

[zvonand]

due to potential caveats

Which caveats do you mean?

[alexey-milovidov]

It can affect the server's logic unrelated to user queries.
It has been demonstrated by randomized testing: #52325

[filimonov]

It has been demonstrated by randomized testing: #52325

Problems of the same nature appear when the server timezone gets changed, indexes with time functions stop working correctly etc. So it's not something new, but once it becomes more dynamic the problem becomes more obvious/visible.