Support NULL time in First/Last/FirstBy/LastBy aggregations #17064
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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).