fix: resolve 15-16 transformer AST mismatch for create_view-281.sql

[pyramation]

Fix: resolve 15-16 transformer AST mismatch for create_view-281.sql

Summary

This PR fixes a critical AST transformation mismatch in the PostgreSQL 15-to-16 transformer that was causing the latest/postgres/create_view-281.sql test case to fail. The issue was related to how SYSTEM_USER references are transformed between PostgreSQL versions.

The root cause was that PostgreSQL 15 and 16 represent SYSTEM_USER differently in their ASTs:

• PG15: Uses ColumnRef for direct references, RangeVar for table-like usage
• PG16: Uses FuncCall with pg_catalog.system_user for both cases, or RangeFunction for table-like usage

The complex CREATE VIEW SQL statement contained multiple forms of SYSTEM_USER usage that required bidirectional transformation logic to handle all cases correctly.

Review & Testing Checklist for Human

• Verify bidirectional transformation logic: The ColumnRef ↔ FuncCall and RangeVar ↔ RangeFunction transformations are complex. Review the logic in both directions to ensure they're consistent and handle edge cases.
• Test the specific SQL query manually: Run the exact SQL from the failing test case through both PG15 and PG16 parsers to verify the transformation produces the expected AST.
• Check for other SYSTEM_USER usage patterns: Search the codebase for other uses of SYSTEM_USER to ensure this fix doesn't break existing functionality.
• Validate empty object handling: The {} as any change in the List method affects many AST nodes. Spot-check a few other transformer tests to ensure no regressions.

---

Diagram

%%{ init : { "theme" : "default" }}%%
graph TB
    subgraph "Test Infrastructure"
        A["transformer-errors.ts"]:::minor-edit
        B["latest-postgres-create_view.test.ts"]:::context
    end
    
    subgraph "Core Transformer"
        C["v15-to-v16.ts"]:::major-edit
        D["ColumnRef method"]:::major-edit
        E["FuncCall method"]:::major-edit
        F["RangeVar method"]:::major-edit
        G["List method"]:::minor-edit
        H["IndexStmt method"]:::minor-edit
    end
    
    subgraph "SQL Query"
        I["CREATE VIEW with SYSTEM_USER"]:::context
        J["Direct SYSTEM_USER references"]:::context
        K["SELECT * FROM SYSTEM_USER"]:::context
    end
    
    A --> C
    B --> C
    C --> D
    C --> E
    C --> F
    C --> G
    C --> H
    I --> J
    I --> K
    J --> D
    J --> E
    K --> F
    
    subgraph Legend
        L1["Major Edit"]:::major-edit
        L2["Minor Edit"]:::minor-edit
        L3["Context/No Edit"]:::context
    end
    
    classDef major-edit fill:#90EE90
    classDef minor-edit fill:#87CEEB
    classDef context fill:#FFFFFF

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Notes

• Session Info: Requested by Dan Lynch (@pyramation) - Devin Session
• Regression Fix: I initially introduced a regression in IndexStmt by accidentally removing nulls_not_distinct handling, which I caught and fixed in a follow-up commit.
• Testing: All 1070 test suites pass, including the previously failing create_view-281.sql test case.
• Risk Assessment: This is a complex transformation with bidirectional logic that requires careful review. The changes affect core AST transformation methods used across many SQL patterns.

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