Prune values across the query executor instead of on the calling thread

[xiangfu0]

What

ValueBasedSegmentPruner now overrides prune(List, QueryContext, ExecutorService) and prunes across the query executor when there are enough segments to be worth it.

It previously implemented only the two-arg prune, so SegmentPruner's three-arg default silently discarded the executor it was handed. BloomFilterSegmentPruner already overrode it and pruned in parallel — the pruner that runs first, over the full segment set, did not.

The parallel path mirrors the bloom-filter one: strided tasks over the segment list, each with its own ValueCache and data-source cache (neither is thread-safe, and both are scoped to a single segment at a time regardless), run through QueryMultiThreadingUtils.runTasksWithDeadline so the query deadline and cancellation are honored. Below TARGET_NUM_SEGMENTS_PER_THREAD, or with no executor, it stays serial.

TARGET_NUM_SEGMENTS_PER_THREAD moves to the parent; BloomFilterSegmentPruner was shadowing it with the same value.

Why

This loop runs on the calling thread for every segment the server holds, before any per-segment parallelism starts. On a server holding 44,780 segments it is the longest single-threaded stretch of the query — and the default pruner chain is ColumnValueSegmentPruner, BloomFilterSegmentPruner, SelectionQuerySegmentPruner, so the serial one runs first and on the largest input.

Stacked on #19511, which removes the per-segment work in that loop (it was materializing a column to read min/max). This removes the serialisation. They are independent: either helps alone, and the combination is what makes pruning proportional to cores rather than to segments held.

Behaviour change

Segments come back in a different order than they went in. That is already true of BloomFilterSegmentPruner, which runs immediately after this pruner in the default chain, so nothing downstream can have depended on the input order surviving pruning.

Tests

ColumnValueSegmentPrunerTest#testParallelPruningSelectsTheSameSegments builds 40 segments — half matching the predicate, half prunable — and asserts the parallel path selects exactly the same set as the serial one. 52 pruner tests pass; spotless, checkstyle and license clean.

Stack

Part 12, based on #19511.

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