Support MERGE INTO

[wirybeaver]

Is your feature request related to a problem or challenge?

The goal is to support MERGE INTO SQL statements in DataFusion so that downstream table providers (specifically Iceberg via iceberg-rust) can implement merge logic. The iceberg-rust repo (feature/merge-into branch) already has a merge_into function on its DataFusion table provider that expects DataFusion to parse MERGE INTO SQL and invoke a merge_into hook on TableProvider.

Example SQL:

MERGE INTO target_table t
USING source_table s
ON t.id = s.id
WHEN MATCHED THEN UPDATE SET t.value = s.value
WHEN NOT MATCHED THEN INSERT (id, value) VALUES (s.id, s.value)

Describe the solution you'd like

The implementation follows the same pattern as UPDATE/DELETE DML hooks (PR#19142). Reuse the existing DmlStatement logical plan node with a new WriteOp::MergeInto(MergeIntoOp) variant. The merge-specific data (ON condition, WHEN clauses) is carried in MergeIntoOp. The DmlStatement.input field holds the source plan (USING clause), and DmlStatement.target holds the target table.

sqlparser v0.61.0 already parses Statement::Merge — no parser work needed.

The following tasks are already implemented in the PoC branch with 3 commits. Plan to raise PRs one by another as the fork repo doesn't support stacking PRs.

• Add MERGE INTO types to datafusion-expr #20763
• Add merge_into hook to TableProvider trait
• Add SQL planner (merge_to_plan) + physical planner dispatch