fix(tsql): Retain limit clause in subquery expression.

[MarcusRisanger]

Fixes issue: TobikoData/sqlmesh#4649

Subqueries are parsed twice (at least for tsql) by select_sql, which causes subqueries using LIMIT or TOP to drop these on the second run, when converting CTAS statement to SELECT .. INTO ...

The subquery expression object suffers a side-effect in Generator.select_sql method where any limit expression is popped from the expression and propagated within the method as a string component.

This simple fix reinjects the original limit expression to any query that HAD a limit expression prior to select_sql, but lacks it afterward.

[georgesittas]

Hey @MarcusRisanger 👋

Thanks a lot for digging into this! I think we may be able to simplify this a bit. I see that the TOP 1 is preserved given this diff:

+++ b/sqlglot/dialects/tsql.py
@@ -1224,7 +1224,6 @@ class TSQL(Dialect):
                     # to amend the AST by moving the CTEs to the CREATE VIEW statement's query.
                     ctas_expression.set("with", with_.pop())

-            sql = super().create_sql(expression)

             table = expression.find(exp.Table)

@@ -1243,6 +1242,8 @@ class TSQL(Dialect):
                     select_into.limit(0, copy=False)

                 sql = self.sql(select_into)
+            else:
+                sql = super().create_sql(expression)

Wanna give it a shot? Feel free to update the PR and I can merge shortly.

[MarcusRisanger]

Very nice @georgesittas - this is the difference between knowing the code base and just blindly implementing change based on debugging call stacks... 🚀