Stop the segment preprocess from building a Schema per segment

[xiangfu0]

What

Two things the segment preprocess does on every load asked the segment metadata for its Schema. The schema is derived and then cached, so each one pinned a Schema per loaded segment for the segment's whole life:

• ForwardIndexHandler#computeOperations wants the set of physical column names.
• ColumnMinMaxValueGenerator wants the columns its mode selects — the default mode is ALL, so this runs on every load too.

Both questions are answered by the column metadata the schema is itself derived from. SegmentMetadata#getPhysicalColumnNames() walks the column metadata (falling back to the schema for a segment that holds no column metadata, i.e. a CONSUMING one), and the min/max generator selects straight off each column's field spec instead of off schema.getAllFieldSpecs().

No behavior change: the same columns are selected, in the same order the column metadata is held in, and getSchema() still returns the same schema for anyone who asks for it.

Why

Found on a production server holding 13.6k loaded segments: one Schema per segment, ~144 MB of tree entries and list slots, all of it a second copy of what the column metadata already holds. Part 7 of this series (#19478) made the per-segment schema lazy; these two callers were the reason it was still built for every segment anyway.

Tests

SegmentMetadataImplTest#testPreprocessDoesNotBuildTheSegmentSchema: getPhysicalColumnNames() returns what getSchema().getPhysicalColumnNames() returns without materializing a schema, and a full SegmentPreProcessor#process() over a v3 segment materializes none. The test fails if either call site is reverted.

Stack

Part 10, based on #19481. Review only this part's own commit; the earlier parts account for the rest of the diff.

1. Allocate ColumnMetadataImpl index sizes lazily and skip the index_map lookup without an index dir #19480 lazy index-size storage
2. Canonicalize the default null value and intern per-column strings at metadata parse time #19473 canonical default-null values and interned per-column strings
3. Delegate immutable DataSourceMetadata to ColumnMetadata instead of snapshotting it #19474 delegating immutable DataSourceMetadata
4. Fold PhysicalColumnIndexContainer's IndexTypeMap into a presence mask and a dense reader array #19475 presence-mask index container
5. Share segment-derived FieldSpec instances across segments through a weak interner #19476 weak FieldSpec interner
6. Materialize immutable-segment columns lazily behind an opt-in instance config (default off) #19477 opt-in lazy column materialization
7. Slim ColumnMetadataImpl to 72 bytes and derive the per-segment Schema lazily #19478 slim ColumnMetadataImpl and lazy per-segment Schema
8. Store numeric column min/max as primitives instead of boxed Comparables #19479 primitive numeric min/max
9. Hold segment column metadata in sorted arrays and derive the map on demand #19481 sorted-array column metadata store
10. this PR — stop the preprocess from building a schema per segment

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.28571% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.79%. Comparing base (f0a20da) to head (1bad6fd).

Files with missing lines	Patch %	Lines
.../columnminmaxvalue/ColumnMinMaxValueGenerator.java	80.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@                           Coverage Diff                            @@
##             xiangfu0/data-3221-9-column-arrays   #19486      +/-   ##
========================================================================
+ Coverage                                 67.76%   67.79%   +0.02%     
  Complexity                                 1430     1430              
========================================================================
  Files                                      3493     3493              
  Lines                                    225070   225067       -3     
  Branches                                  35550    35545       -5     
========================================================================
+ Hits                                     152515   152578      +63     
+ Misses                                    60517    60450      -67     
- Partials                                  12038    12039       +1     

Flag	Coverage Δ
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-25	67.79% <89.28%> (+0.02%)	⬆️
lane-a	100.00% <ø> (ø)
lane-b	0.00% <ø> (ø)
temurin	67.79% <89.28%> (+0.02%)	⬆️
unittests	67.78% <89.28%> (+0.02%)	⬆️
unittests1	57.80% <78.57%> (+0.01%)	⬆️
unittests2	39.47% <46.42%> (+0.02%)	⬆️

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