Support MySQL system variables

[qgallet]

Brief summary of the change made

Fixes #3574
MySQL system variables @@global.xxx and @@session.xxx are now recognized by the parser.

Are there any other side effects of this change that we should be aware of?

None that I can think of.

Pull Request checklist

• Please confirm you have completed any of the necessary steps below.

• Included test cases to demonstrate any code changes, which may be one or more of the following:

  • .yml rule test cases in test/fixtures/rules/std_rule_cases.
  • .sql/.yml parser test cases in test/fixtures/dialects (note YML files can be auto generated with tox -e generate-fixture-yml).
  • Full autofix test cases in test/fixtures/linter/autofix.
  • Other.

• Added appropriate documentation for the change.

• Created GitHub issues for any relevant followup/future enhancements if appropriate.

[barrywhart]

I think . here means "match any character. Should this be \.?

As written, I think it will also allow the name to begin and/or end with a .. That worries me a little. Can we prevent this?

[qgallet]

I double checked, metacharacters (except \) are matched literally inside a character class in Python regex. This fails as expected on a variable with an invalid char in the name : echo 'SELECT @var&broken | sqlfluff parse - --dialect mysql.

For your second point, this indeed looks wrong, but I've found MySQL is so bizarre this is actually valid. The documentation states:

User variables are written as @var_name, where the variable name var_name consists of alphanumeric characters, ., _, and $. A user variable name can contain other characters if you quote it as a string or identifier (for example, @'my-var', @"my-var", or @`my-var`).

I hardly believe anyone in their right mind would use a variable name with a . at the start or end, so I'm happy to change it to something like r"@@?[a-zA-Z0-9_]+\.?[a-zA-Z0-9_]*". The $ sign should be added as well, just let me know what's preferable for the tool and I'll prepare the commit

[barrywhart]

The above looks close. I think you want the dot and the remainder to look like ‘(.[a-zA-Z0-9_]+)?`, to prevent it ending with a dot.

[qgallet]

Alright, I've pushed the change with the $ sign also supported, and added more fixtures for session variables. Thanks for the feedback!

[codecov]

Codecov Report

Merging #3576 (f9a071b) into main (50f0638) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main     #3576   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          174       174           
  Lines        13144     13144           
=========================================
  Hits         13144     13144           

Impacted Files	Coverage Δ
src/sqlfluff/dialects/dialect_mysql.py	100.00% <ø> (ø)

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[barrywhart]

Looks great, thanks!