feat: optimize count(*) for pk/append table

[lucasfang]

Purpose

Linked issue: close #321

This PR adds an optimized count(*) path for both PK and Append tables by introducing a dedicated CountReader API and table-specific implementations.

Main changes:

• Add CountReader API and TableRead::CreateCountReader(splits).
• Implement PK count path via PKCountReader (metadata path + MOR merge path).
• Implement Append count path via AppendCountReader (metadata-first).
• Keep current unsupported boundaries explicit:
  • predicate pushdown is not supported in CountReader path
  • force_keep_delete is not supported in PK CountReader path

Tests

UT:

• Add src/paimon/core/table/source/pk_count_reader_test.cpp
  • MOR snapshot count
  • DV snapshot count
  • empty splits
  • invalid split type
• Add src/paimon/core/table/source/append_count_reader_test.cpp
  • append snapshot count
  • empty splits
  • invalid split type
  • predicate-not-supported behavior

IT:

• Update test/inte/scan_and_read_inte_test.cpp
  • merge count assertions into corresponding original PK scan/read tests
  • keep consistency check between CreateReader row count and CreateCountReader result
  • keep unsupported-option tests

API and Format

API impact:

• Add include/paimon/reader/count_reader.h
• Add TableRead::CreateCountReader(splits) in include/paimon/table/source/table_read.h

Storage format / protocol impact:

• No storage format change
• No protocol compatibility change

Documentation

No additional user-facing documentation is required for this PR.

Generative AI tooling

Generated-by: GitHub Copilot (GPT-5.3-Codex)

[Copilot]

Pull request overview

Adds an optimized CountReader API for PK tables. The default TableRead::CreateCountReader returns NotImplemented, while KeyValueTableRead provides a real implementation built on a new CountSplitRead that picks between a metadata-only count for raw-convertible/DV splits and a merge + drop-delete + count path (via a new RowCountAccumulator) for MOR splits, using a column-pruned read schema. Predicate pushdown and force_keep_delete are rejected for now. Integration tests verify counts against existing full-read results, plus empty-splits and unsupported-option cases.

Changes:

• New CountReader abstraction (include/paimon/reader/count_reader.h) and TableRead::CreateCountReader with PK implementation via CountSplitRead + RowCountAccumulator.
• Metadata-based counting reuses DataSplitImpl::PartialMergedRowCount; MOR path reuses MergeFileSplitRead::CreateSortMergeReaderForSection(..., drop_delete=true) with a trimmed PK-only schema (added InternalReadContext::CreateWithSchema and MergeFileSplitRead::GetKeyComparator).
• Adds count integration tests and an unrelated NormalizePath scheme test.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
include/paimon/reader/count_reader.h	New CountReader interface with CountRows().
include/paimon/table/source/table_read.h	Adds virtual CreateCountReader(splits) on TableRead.
src/paimon/core/table/source/table_read.cpp	Default CreateCountReader returns NotImplemented.
src/paimon/core/table/source/key_value_table_read.{h,cpp}	PK CreateCountReader implementation; rejects predicate and force_keep_delete; stores path factory/context/executor.
src/paimon/core/operation/count_split_read.{h,cpp}	New CountSplitRead with metadata-count + merge-count strategies and PK-only read schema.
src/paimon/core/mergetree/row_count_accumulator.{h,cpp}	New helper that iterates SortMergeReader output and counts rows.
src/paimon/core/operation/internal_read_context.{h,cpp}	Adds CreateWithSchema factory to clone context with a new read schema.
src/paimon/core/operation/merge_file_split_read.h	Exposes GetKeyComparator() for use by count path.
src/paimon/CMakeLists.txt	Registers the two new source files.
test/inte/scan_and_read_inte_test.cpp	Adds count IT cases (MOR/DV/empty/consistency/predicate/force-keep-delete).
src/paimon/common/utils/path_util_test.cpp	Unrelated test for NormalizePath with URI scheme.

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[lxy-9602]

Please add a issue for this pr.

[lxy-9602]

+1