feat: scaffold plpgsql-deparser package for PL/pgSQL AST deparsing

[pyramation]

feat: scaffold plpgsql-deparser package for PL/pgSQL AST deparsing

Summary

This PR adds a new plpgsql-deparser package that converts PL/pgSQL function ASTs back to SQL strings. The PL/pgSQL AST (returned by parsePlPgSQL from @libpg-query/parser) is structurally different from the regular SQL AST and represents the internal structure of PL/pgSQL function bodies.

Background: Investigation confirmed that @libpg-query/parser (libpg-query-node's full/ package) exports parsePlPgSQL and parsePlPgSQLSync functions, but pgsql-parser does not currently re-export these. This package provides the deparsing counterpart for PL/pgSQL ASTs.

Supported constructs include:

• Variable declarations (DECLARE section with types, defaults, CONSTANT, NOT NULL)
• Control flow (IF/ELSIF/ELSE, CASE, LOOP, WHILE, FOR, FOREACH)
• Exception handling (BEGIN...EXCEPTION...END blocks)
• Cursor operations (OPEN, FETCH, CLOSE, MOVE)
• Return statements (RETURN, RETURN NEXT, RETURN QUERY)
• Dynamic SQL (EXECUTE with USING)
• RAISE statements with all log levels
• And 30+ other PL/pgSQL statement types

Updates since last revision

• Fixed CALL statement deparser: Removed duplicate CALL keyword - the parsed expression already contains CALL from the parser
• Improved loop variable detection: Now only excludes implicitly declared loop variables (where var.lineno === stmt.lineno). Explicitly declared variables in DECLARE sections are preserved even when used in FOR loops.
• Added plpgsql-pretty fixtures: Created __fixtures__/plpgsql-pretty/ with sample SQL files (simple-function.sql, if-else-function.sql, loop-function.sql)
• Added snapshot testing infrastructure:
  • PlpgsqlPrettyTest utility class for generating snapshot tests
  • Snapshot tests for uppercase/lowercase formatting modes
  • 12 tests now pass (6 round-trip + 6 snapshot tests)

Review & Testing Checklist for Human

• Verify loop variable lineno matching logic - The fix uses var.lineno === stmt.lineno to detect implicitly declared loop variables. This heuristic could fail with same-line declarations or minified SQL. Check collectLoopVariables() in plpgsql-deparser.ts.
• Test CALL statement edge cases - The fix assumes the parsed expression always contains the CALL keyword. Verify this holds for all CALL statement variants.
• Review snapshot test coverage - The snapshot tests only cover uppercase/lowercase formatting. The deparser doesn't have a "pretty" mode yet, so these test formatting consistency rather than pretty-printing.
• Verify round-trip testing catches real issues - Only plpgsql_domain fixtures pass full round-trip testing. Other categories (plpgsql_control, plpgsql_transaction) still fail due to cursor declarations and complex SQL expression handling.

Recommended test plan:

1. Run pnpm install and pnpm test in packages/plpgsql-deparser/
2. Verify 12 tests pass (6 fixture tests + 6 snapshot tests)
3. Run npm run fixtures to verify fixture generation works
4. Test a function with explicit DECLARE + FOR loop using same variable to verify the lineno fix

Notes

• This is a scaffolding PR - the deparser is functional but has known issues with cursor declarations and complex SQL expressions that cause some round-trip tests to fail
• The round-trip testing framework is working correctly and catching real deparser bugs
• The package does not integrate with pgsql-parser exports; users need to use @libpg-query/parser directly for parsing
• Link to Devin run: https://app.devin.ai/sessions/50192c7bb0454b26845853867bd795ce
• Requested by: Dan Lynch (@pyramation)

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