Dolt differs from MySQL on allowing query global only system variable with session scope

Description

mysql> SELECT @@system_time_zone;
+----------------------------+
| @@SESSION.system_time_zone |
+----------------------------+
| PDT                        |
+----------------------------+
1 row in set (0.00 sec)

mysql> SELECT @@SESSION.system_time_zone;
+----------------------------+
| @@SESSION.system_time_zone |
+----------------------------+
| PDT                        |
+----------------------------+
1 row in set (0.00 sec)

mysql> SELECT @@system_time_zone;
+--------------------+
| @@system_time_zone |
+--------------------+
| PDT                |
+--------------------+
1 row in set (0.00 sec)

mysql> SELECT @@SESSION.system_time_zone;
ERROR 1238 (HY000): Variable 'system_time_zone' is a GLOBAL variable

Contributor guide

Tech stack

gosqlmysql

Domain

database

Issue type

bug

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

3

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

1-3 hours

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

active

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

MySQL system variablesGo programming

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

60

Research direction

Investigate Dolt's system variable handling, especially for global only variables. Look at how scope is determined in SQL parsing and variable lookup. The fix should match MySQL's behavior: throw error when querying global only variable with SESSION scope, and display correct header without scope. Check files like sql/variables.go or similar.