MDEV-40672: Pluggable Aggregate Function

[drrtuy]

Add pluggable aggregate function support

What

Implement MDEV-40672 by extending function plugins to provide aggregate functions through the standard Item_sum lifecycle. Support grouped aggregation, DISTINCT, window functions, native and pluggable data types, and safe plugin lifetime management.

Key changes

• Extend Plugin_function descriptors to distinguish scalar and aggregate functions.
• Add Item_sum_plugin as the base class for plugin-provided aggregates.
• Support DISTINCT argument replay and pluggable result types such as UUID.
• Enable plugin aggregates as window functions, including moving-frame removal.
• Add test aggregates and MTR coverage for grouping, windows, prepared statements, plugin unloading, invalid descriptors, and type preservation.

How to test

Run:

/git/BuildOf_mdb-13/mysql-test/mtr function_plugin function_plugin_extra

Both tests pass.

[Copilot]

Pull request overview

This PR implements MDEV-40672 by extending the function plugin framework so plugins can provide aggregate functions that participate in the standard Item_sum lifecycle (grouped aggregation, DISTINCT handling, and window-function execution), with accompanying func_test plugin implementations and MTR coverage.

Changes:

• Extend the SQL grammar to recognize generic-function aggregates (including plugin aggregates) and allow DISTINCT in the generic function-call path.
• Add PLUGIN_SUM_FUNC / Item_sum_plugin plus distinct-argument replay support via a new Aggregator::arg_item() API.
• Add aggregate plugin test functions (test_plugin_first, test_plugin_count) and MTR tests covering grouping, distinct, windows, and type preservation.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
sql/sql_yacc.yy	Parser updates to detect aggregate-capable generic functions, support DISTINCT, and allow plugin aggregates in window-function contexts.
sql/sql_window.cc	Reject plugin aggregates with DISTINCT when used as window functions (explicit ER_NOT_SUPPORTED_YET).
sql/item_sum.h	Introduces PLUGIN_SUM_FUNC, Item_sum_plugin, and adds Aggregator::arg_item() to support DISTINCT replay for plugin aggregates.
sql/item_sum.cc	Implements Item_sum_plugin::fix_fields() and extends distinct-aggregator logic to replay distinct values for plugin aggregates.
sql/item_create.h	Adds Create_aggregate_func marker type to distinguish scalar vs aggregate native/plugin builders.
plugin/func_test/plugin.cc	Adds two aggregate function plugins used by tests (test_plugin_first, test_plugin_count).
plugin/func_test/mysql-test/func_test/function_plugin.test	New MTR test covering plugin aggregates (grouping, DISTINCT, windows, UUID preservation, etc.).
plugin/func_test/mysql-test/func_test/function_plugin.result	Expected results for function_plugin.test.
plugin/func_test/mysql-test/func_test/function_plugin_scalar_unload.test	New test intended to demonstrate scalar plugin unload race (currently lacks .result and is described as crashing on unfixed servers).
plugin/func_test/mysql-test/func_test/function_plugin_negative.test	Negative-coverage MTR test for invalid contexts/usages and error paths.
plugin/func_test/mysql-test/func_test/function_plugin_negative.result	Expected results for function_plugin_negative.test.
plugin/func_test/mysql-test/func_test/function_plugin_extra.test	Additional MTR coverage for distinct, spill-to-disk, pluggable types, and reprepares.
plugin/func_test/mysql-test/func_test/function_plugin_extra.result	Expected results for function_plugin_extra.test.
include/mysql/plugin_function.h.pp	Forward-declares Create_func to keep generated header consistent with updated API.
include/mysql/plugin_function.h	Documents aggregate plugin requirements and forward-declares Create_func.

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