feat(parser): ClickHouse WITH TOTALS / LIMIT BY / ANY JOIN / DEFAULT ident (#482)

[ajitpratap0]

Summary

Four ClickHouse parser fixes from the QA residuals (#482): WITH TOTALS, LIMIT BY, ANY/ALL JOIN prefix, DEFAULT/DATABASES as identifiers.

Test plan

• 11 test shapes across 4 test functions, race-tested green

Part of #482.

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Project	Deployment	Actions	Updated (UTC)
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PR Review: ClickHouse Parser Fixes

Comprehensive review of the ClickHouse parser improvements for WITH TOTALS, LIMIT BY, ANY/ALL JOIN prefix, and DEFAULT/DATABASES identifier support.

Todo List:

• Read repository guidelines (CLAUDE.md)
• Analyze parser.go changes
• Review select.go modifications
• Examine select_clauses.go updates
• Review new test file (clickhouse_misc_fixes_test.go)
• Evaluate code quality and best practices
• Check for potential bugs or issues
• Assess performance considerations
• Review security concerns
• Evaluate test coverage
• Provide final feedback and recommendations

✅ Overall Assessment: LGTM with Minor Suggestions

This PR implements targeted ClickHouse parser fixes with clean, focused changes. The implementation follows repository patterns and maintains production quality standards.

Strengths

Code Quality & Architecture

• Consistent dialect-specific guards: All changes properly use p.dialect == string(keywords.DialectClickHouse) checks
• Minimal invasive approach: Changes are surgically precise, affecting only ClickHouse-specific parsing paths
• Follows existing patterns: Matches the repository's established parser architecture and coding conventions
• Clean separation of concerns: Each feature is implemented in its appropriate parser method

Memory Management & Performance

• Zero allocations added: No new object creation or pooling concerns
• Efficient token consumption: Uses existing token advancement patterns without backtracking
• Minimal overhead: Dialect checks are O(1) string comparisons that won't impact hot paths

Test Coverage

• Comprehensive test shapes: 11 test cases across 4 test functions covering all features
• Realistic SQL patterns: Tests use practical ClickHouse query patterns
• Proper dialect testing: All tests correctly use gosqlx.ParseWithDialect()

Implementation Review

1. DEFAULT/DATABASES as Identifiers (parser.go:1066, 1076)

// Added TokenTypeDefault to non-reserved keywords
models.TokenTypePartition, models.TokenTypeRows, models.TokenTypeDefault:

// Added "DATABASES" to permissive identifier list
"TABLES", "DATABASES":

✅ Well implemented: Follows established pattern for DDL keywords used as identifiers

2. ANY/ALL JOIN Prefix (parser.go:1115-1119, select_clauses.go:185-192)

// Join detection logic
if p.dialect == string(keywords.DialectClickHouse) &&
    (p.isType(models.TokenTypeAny) || p.isType(models.TokenTypeAll)) {
    return true
}
// Consumption in parseJoinType
if p.dialect == string(keywords.DialectClickHouse) &&
    (p.isType(models.TokenTypeAny) || p.isType(models.TokenTypeAll)) {
    p.advance()
}

✅ Correct implementation: Properly handles ClickHouse join strictness modifiers

3. LIMIT BY Clause (select.go:182-198)

if p.dialect == string(keywords.DialectClickHouse) && p.isType(models.TokenTypeBy) {
    p.advance() // Consume BY
    for {
        if _, err := p.parseExpression(); err != nil {
            return nil, err
        }
        if !p.isType(models.TokenTypeComma) {
            break
        }
        p.advance()
    }
}

✅ Robust parsing: Handles comma-separated expression lists with proper error handling

4. WITH TOTALS (select_clauses.go:502-508)

case "TOTALS":
    // ClickHouse WITH TOTALS: adds a summary row with aggregate totals.
    // Consumed but not modeled on the AST (follow-up).
    p.advance() // Consume WITH
    p.advance() // Consume TOTALS

✅ Pragmatic approach: Correctly consumes tokens while noting AST modeling as future work

Security Assessment

• No injection vectors: All features consume tokens through established parser methods
• Proper bounds checking: Expression parsing includes error handling
• Dialect isolation: ClickHouse-specific features are properly isolated from other dialects

Minor Suggestions

1. Error Handling Enhancement

The LIMIT BY parsing could benefit from more specific error messages:

// Consider adding context to errors
if _, err := p.parseExpression(); err != nil {
    return nil, fmt.Errorf("parsing LIMIT BY expression: %w", err)
}

2. Test Enhancement Opportunity

Consider adding edge case tests:

// Test empty BY clause handling
"SELECT * FROM t LIMIT 1 BY"  // Should error gracefully
// Test complex expressions
"SELECT * FROM t LIMIT 1 BY func(col), col + 1"

3. Documentation Note

The code comments are excellent, but consider adding the ClickHouse documentation references for future maintainers.

Performance Considerations

• Parsing overhead: Negligible impact - all additions are behind dialect checks
• Memory usage: No additional memory allocations
• Compatibility: Changes don't affect other SQL dialects

Technical Correctness

All implementations align with ClickHouse SQL syntax:

• LIMIT N BY expression - ✅ Correct
• GROUP BY ... WITH TOTALS - ✅ Correct
• ANY LEFT JOIN / ALL INNER JOIN - ✅ Correct
• DEFAULT and DATABASES as identifiers - ✅ Correct

Final Verdict

This PR demonstrates excellent engineering practices with focused, well-tested improvements to ClickHouse parser support. The changes are production-ready and maintain the repository's high standards for performance and reliability.

Recommended action: ✅ Approve and merge

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