refactor(btree): eliminate hot-path overhead from allocs and deserialization

[lxy-9602]

Purpose

Linked issue: #38

Optimize BTree global index read performance by reducing memory allocations and eliminating unnecessary deserialization in the hot path.

Changes

• MemorySegment: add non-owning WrapView mode with cached data_/size_, avoiding shared_ptr dereference on every access.
• MemorySlice: ReadSlice now returns a zero-copy non-owning view.
• VarLengthIntUtils: unify varint encode/decode into static inline functions with 1-byte fast path for values 0–127.
• MemorySliceInput: inline all hot-path methods into header; delete memory_slice_input.cpp.
• BTreeGlobalIndexReader: compare keys at byte level via SliceComparator instead of deserializing to Literal per entry.
• KeySerializer: add fast-path comparator for integer and string.
• BlockIterator: return BlockEntry by value instead of unique_ptr.

Tests

MemorySegmentTest.TestNonOwningWrapView

API and Format

Documentation

Generative AI tooling

Partially Generated-by: Claude-4.6-Opus

[Copilot]

Pull request overview

Optimizes hot-path performance for BTree global index reads by reducing allocations, introducing non-owning memory views, and avoiding per-entry key deserialization during range scans.

Changes:

• Introduce non-owning MemorySegment::WrapView and propagate view-based APIs (MemorySlice::Data, MemorySliceInput::ReadSlice zero-copy).
• Refactor varint encoding/decoding into VarLengthIntUtils functions operating on raw buffers and inline hot-path MemorySliceInput.
• Speed up BTree range queries by comparing serialized keys via a MemorySlice::SliceComparator instead of deserializing to Literal per entry; adjust SST block iteration to return BlockEntry by value.

Reviewed changes

Copilot reviewed 29 out of 29 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/paimon/core/mergetree/lookup/persist_position_processor.h	Switch varint encode/decode calls to raw-buffer APIs.
src/paimon/common/utils/var_length_int_utils_test.cpp	Update varint tests to use stack buffers and new APIs.
src/paimon/common/utils/var_length_int_utils.h	Rework varint utilities to encode/decode from char* with a 1-byte decode fast path.
src/paimon/common/sst/sst_file_reader.cpp	Adapt to BlockIterator::Next() returning BlockEntry by value; use MemorySegment::Data/Size/MutableData.
src/paimon/common/sst/block_iterator.h	Change iterator API to return BlockEntry by value.
src/paimon/common/sst/block_iterator.cpp	Implement value-returning Next/ReadEntry.
src/paimon/common/sst/block_entry.h	Remove now-unneeded <memory> include; keep BlockEntry as slices.
src/paimon/common/sst/block_cache_test.cpp	Update test to use new segment heap accessor naming.
src/paimon/common/sst/block_cache.h	Read blocks via MemorySegment::MutableData().
src/paimon/common/memory/memory_slice_test.cpp	Rename test to match new heap accessor API.
src/paimon/common/memory/memory_slice_output.cpp	Encode varints via VarLengthIntUtils into the output buffer.
src/paimon/common/memory/memory_slice_input.h	Inline hot-path methods; make ReadSlice return a non-owning view.
src/paimon/common/memory/memory_slice_input.cpp	Remove out-of-line implementation (file deleted).
src/paimon/common/memory/memory_slice.h	Add Data() accessor; rename heap accessor to GetOrCreateHeapMemory().
src/paimon/common/memory/memory_slice.cpp	Implement GetOrCreateHeapMemory(), ReadStringView() via Data(), and faster CopyBytes().
src/paimon/common/memory/memory_segment_utils.cpp	Update to GetOrCreateHeapMemory() naming/behavior.
src/paimon/common/memory/memory_segment_test.cpp	Add coverage for non-owning WrapView behavior.
src/paimon/common/memory/memory_segment.h	Add WrapView, cached data_/size_, and Data()/MutableData() accessors; rename heap accessor.
src/paimon/common/memory/memory_segment.cpp	Remove SwapBytes implementation.
src/paimon/common/io/cache/lru_cache_test.cpp	Use MemorySegment::MutableData() for fills.
src/paimon/common/global_index/btree/key_serializer.cpp	Add fast-path MemorySlice comparators to avoid Literal deserialization.
src/paimon/common/global_index/btree/btree_index_meta.cpp	Use GetOrCreateHeapMemory() for serialized output.
src/paimon/common/global_index/btree/btree_global_index_reader.h	Store a pre-built slice comparator for range queries.
src/paimon/common/global_index/btree/btree_global_index_reader.cpp	Compare keys via SliceComparator and serialize bounds once per query.
src/paimon/common/data/serializer/row_compacted_serializer.h	Decode varints from MemorySegment::Data().
src/paimon/common/data/serializer/row_compacted_serializer.cpp	Encode varints into raw buffers; read strings via segment_.Data().
src/paimon/common/data/binary_string.cpp	Read string views via MemorySegment::Data().
src/paimon/common/data/abstract_binary_writer.cpp	Grow by copying from GetOrCreateHeapMemory() instead of direct array access.
src/paimon/CMakeLists.txt	Remove memory_slice_input.cpp from build.

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