Support NULL time in First/Last/FirstBy/LastBy aggregations

[alpass163]

Description:

This PR introduces robust support for NULL timestamps in FIRST, LAST, FIRST_BY, and LAST_BY aggregation functions. Previously, rows with NULL time might have been handled inconsistently or ignored.

Key Changes:

Null Time Handling Strategy:

Priority: Data with a Valid Time always takes precedence over data with a Null Time.

Fallback Policy: If only NULL time data exists, the aggregations now adopt a "First Null Wins" strategy. This means the first encountered value with a NULL timestamp is retained as the result, ensuring consistent behavior across both First and Last families.

Comprehensive Support:

Applied these changes to both standard Accumulators (for global aggregation) and GroupedAccumulators (for GROUP BY queries).

Covered all supported data types (INT32, INT64, FLOAT, DOUBLE, BINARY, BOOLEAN).

[Copilot]

Pull request overview

This pull request adds comprehensive support for NULL timestamps in FIRST, LAST, FIRST_BY, and LAST_BY aggregation functions. The implementation prioritizes data with valid timestamps over NULL timestamp data, and when only NULL timestamp data exists, adopts a "First Null Wins" strategy where the first encountered NULL time value is retained.

Changes:

• Added initNullTimeValues tracking field to all grouped accumulators and initNullTimeValue to non-grouped accumulators
• Enhanced serialization format to include an additional boolean flag indicating whether the order time is NULL
• Implemented separate update methods for NULL time values across all data types (INT32, INT64, FLOAT, DOUBLE, BINARY, BOOLEAN)

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
GroupedLastByAccumulator.java	Added NULL time tracking with initNullTimeValues field and separate update paths for NULL vs valid timestamps
GroupedLastAccumulator.java	Similar NULL time support for LAST aggregation with dual initialization tracking
GroupedFirstByAccumulator.java	Mirrored LAST logic but with minimum time comparison for FIRST semantics
GroupedFirstAccumulator.java	NULL time handling for non-BY FIRST aggregation
Utils.java	New serialization methods serializeTimeValueWithNull to handle the extended format with NULL time flags
LastDescAccumulator.java	Descending order variant with NULL time value handling
LastByDescAccumulator.java	Descending LAST_BY with NULL time support
LastByAccumulator.java	Non-grouped LAST_BY with NULL time tracking
LastAccumulator.java	Non-grouped LAST with NULL time support
FirstDescAccumulator.java	Descending FIRST with NULL time handling
FirstByDescAccumulator.java	Descending FIRST_BY with NULL time support
FirstByAccumulator.java	Non-grouped FIRST_BY with NULL time tracking
FirstAccumulator.java	Non-grouped FIRST with comprehensive NULL time support

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