Implement basic Aggregate Merge Engine

[platinumhamburg]

Purpose

Linked issue: close #2254

Brief change log

Tests

API and Format

Documentation

[Copilot]

Pull request overview

This PR implements the Aggregation Merge Engine for Fluss, which allows field-level aggregation of rows with the same primary key. The implementation includes 12 aggregate functions (sum, product, max, min, last_value, last_value_ignore_nulls, first_value, first_value_ignore_nulls, listagg, string_agg, bool_and, bool_or) with comprehensive schema evolution support.

Key changes:

• Core aggregation engine with field-level aggregators and caching
• Schema API enhancements to support aggregation function configuration
• Comprehensive documentation with examples for all supported functions
• Utility classes for string concatenation and value comparison

Reviewed changes

Copilot reviewed 56 out of 56 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
website/docs/table-design/table-types/pk-table/merge-engines/aggregation.md	Comprehensive documentation for the Aggregation Merge Engine with examples
fluss-server/src/main/java/org/apache/fluss/server/kv/rowmerger/AggregateRowMerger.java	Main row merger implementation with partial update support
fluss-server/src/main/java/org/apache/fluss/server/kv/rowmerger/aggregate/*.java	Aggregation context, caching, and field processing logic
fluss-server/src/main/java/org/apache/fluss/server/kv/rowmerger/aggregate/functions/*.java	Field aggregator implementations for all functions
fluss-server/src/main/java/org/apache/fluss/server/kv/rowmerger/aggregate/factory/*.java	Factory classes for creating aggregators via SPI
fluss-common/src/main/java/org/apache/fluss/metadata/AggFunction.java	Enum defining all supported aggregation functions
fluss-common/src/main/java/org/apache/fluss/metadata/Schema.java	Schema enhancements to support aggregation functions
fluss-common/src/main/java/org/apache/fluss/utils/*.java	Utility classes for string operations and comparisons
fluss-common/src/main/java/org/apache/fluss/config/ConfigOptions.java	New configuration options for aggregation
Test files	Comprehensive test coverage for all components

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[wuchong]

Thanks @platinumhamburg , I didn't complete the review. I gave some feedbacks first after reading the documentation.

[platinumhamburg]

Thanks @platinumhamburg , I didn't complete the review. I gave some feedbacks first after reading the documentation.

@wuchong I've refactored both the code and documentation based on the feedback. Thanks for review again.