feat: Implements basic snowflake session variables via SET/UNSET

[jsibbison-square]

Implements the very basic usages of sql variables

SET var2 = 'blue';
SELECT * from table where color = $var2;                  Produces: SELECT * from table where color = 'blue';
UNSET var2;

as described in https://docs.snowflake.com/en/sql-reference/session-variables.

List of features supported and not supported by this PR:

• Variables are scoped to the session (Eg. The connection, not the cursor)
• Simple scalar variables: SET var1 = 1;
• Unset variables: UNSET var1;
• Simple SQL expression variables: SET INCREMENTAL_DATE = DATEADD( 'DAY', -7, CURRENT_DATE());
• Basic use of variables in SQL using $ syntax: SELECT $var1;
• Multiple variables: SET (var1, var2) = (1, 'hello');
• Variables set via 'properties' on the connection https://docs.snowflake.com/en/sql-reference/session-variables#setting-variables-on-connection
• Using variables via the IDENTIFIER function: INSERT INTO IDENTIFIER($my_table_name) (i) VALUES (42);
• Session variable functions: https://docs.snowflake.com/en/sql-reference/session-variables#session-variable-functions

[tekumara]

This is a great feature, thanks for implementing this! Some suggestions (including optional ones) below

[tekumara]

The tests should pass on a real Snowflake instance to confirm they correctly match Snowflake's behaviour. Try running this on Snowflake. When I do, for these two lines, I get the error:

ProgrammingError: 000002 (0A000): Unsupported feature 'assignment from non-constant source expression'.

Does that happen for you?

This is what works for me:

Suggested change

	cur.execute('SET var3 = parse_json(\'{"k1": "v1"}\');')
	cur.execute("SET var4 = array_construct(1, 2, 3);")
	cur.execute('SET var3 = (select to_json(parse_json(\'{"k1": "v1"}\')));')
	cur.execute("SET var4 = (select to_json(array_construct(1, 2, 3)));")

[jsibbison-square]

Can't believe I missed this initially 🤦 .

[tekumara]

To match Snowflake:

Suggested change

	assert cur.fetchall() == [(1, "hello", '{"k1":"v1"}', [1, 2, 3], datetime.date.today())]
	assert cur.fetchall() == [(1, "hello", '{"k1":"v1"}', '[1,2,3]', datetime.date.today())]

[tekumara]

To test for an exception I'd use pytest.raises, eg:

Suggested change

	try:
	cur.execute("select $var3;")
	raise AssertionError("Expected exception")
	except duckdb.duckdb.InvalidInputException:
	pass
	with pytest.raises(ProgrammingError, match='Session variable '$VAR3' does not exist'):
	cur.execute("select $var3;")

Note also fakesnow should raise the snowflake ProgrammingError rather than a duckdb error, to avoid the duckdb implementation leaking out into clients.

And optionally have a similar error message to Snowflake eg:

Session variable '$VAR3' does not exist

[tekumara]

Suggested change

	assert cur.fetchall() == [(1, "hello", [1, 2, 3])]
	assert cur.fetchall() == [(1, "hello", '[1,2,3]')]

[tekumara]

(optional) for consistency I think this could be handled like any other transform, eg: in transforms.py as the method

def update_variables(
    expression: exp.Expression,
    variables: dict[str, Any],
) -> exp.Expression:
    # if is variable modifier, update variables dict and return transforms.SUCCESS_NOP

[jsibbison-square]

These comments make sense. I'll work through these changes and notify you when its ready for review again.

[jsibbison-square]

@tekumara I've made updates from all the comments I believe this is ready for review again :).

[tekumara]

Prefect thank you!