feat(converters): add native ClickHouse SQL dialect converter (#546)

[MahmoudHassanen99]

Summary

This PR introduces a native ClickHouse SQL dialect converter under converters/clickhouse/ as proposed in #546.

Key additions:

• Aggregate Mappings: Mapped native ClickHouse combinator functions (uniqExact, uniqCombined64, quantileExact).
• Time Grains: Mapped temporal truncation functions (toStartOfDay, toStartOfWeek, toStartOfMonth, toStartOfQuarter, toStartOfYear).
• Safe Division: Applied nullIf(divisor, 0) guards to prevent division-by-zero exceptions.
• Tests & Quality: Added full unit test suite under converters/clickhouse/tests/ with 100% test coverage and validated query syntax against a live ClickHouse engine.

Related Issues

Closes #546

Checklist

Specification

• Spec changes are included in core-spec/ and follow the existing structure
• Spec changes have been discussed on the mailing list or in a linked issue
• Breaking changes to the spec are clearly called out in the summary

Ontology

• Ontology changes in ontology/ are consistent with spec changes
• New or modified terms are defined and documented

Converters

• Converter logic in converters/ is updated to reflect spec or ontology changes
• New converters include tests under the converter's test directory
• If adding a new converter, .github/labeler.yml is updated with the new path

Validation

• Validation rules in validation/ are updated if the spec changed
• New validation cases are covered by tests

Documentation

• docs/ is updated to reflect any user-facing changes
• New features or behaviors are documented with examples where appropriate
• CONTRIBUTING.md is updated if the contribution process changed

Examples

• examples/ are added or updated for any new spec constructs or converter support

Tests

• All existing tests pass (pytest / CI green)
• New functionality is covered by tests

Compliance

• ASF license headers are present on all new source files
• No third-party dependencies are added without PMC/IPMC approval
• If third-party source code is vendored/copied (not just declared as a dependency), NOTICE and/or LICENSE have been updated per ASF policy

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[ekut]

Thanks for the PR. One semantic issue in the aggregate mappings, and a suggestion for the tests.

median and percentile both map to quantileExact, which returns an actual value from the group and does not interpolate. The spec defines PERCENTILE_CONT as interpolated and MEDIAN as the 50th percentile [1]. Over the values 1, 2, 3, 4 on ClickHouse 26.9.5.2:

• quantileExact(0.5)(x) returns 3
• quantileExactInclusive(0.5)(x) returns 2.5, as do PERCENTILE_CONT(0.5) and MEDIAN in DuckDB 1.5.6

quantileExactInclusive matched DuckDB's PERCENTILE_CONT in all 63 combinations of group size and level I tried. On an empty group it returns nan; quantileExactInclusiveOrNull returns NULL there. If percentile is meant as PERCENTILE_DISC, none of quantileExact, quantileExactLow or quantileExactHigh matched it in every case. The value at position max(1, ceil(level * n)) of the sorted group did.

The tests compare generated SQL strings, so they pass with either function. Running each mapping on a small fixture in ClickHouse and comparing the result with a reference engine such as DuckDB would catch this.

[1] https://github.com/apache/ossie/blob/main/core-spec/expression_language.md#percentile-functions-required

[MahmoudHassanen99]

Thanks for catching this @ekut, You're completely right
quantileExactInclusive matches the continuous percentile (PERCENTILE_CONT) interpolation required by the Ossie spec.
I have updated both median and percentile mappings to use quantileExactInclusive and updated the corresponding test suite. Pushed the fix in the latest commit