[wirybeaver]

Which issue does this PR close?

Follow-up to the MERGE INTO review in #22988 and split from #24195. No separate issue.

Rationale for this change

Logical-plan output for MERGE INTO currently shows only op=[MergeInto]. It omits the ON condition and WHEN actions, which makes EXPLAIN less useful for understanding or debugging a merge plan.

What changes are included in this PR?

• Adds display implementations for MERGE operations, clause kinds, clauses, and actions.
• Includes the ON condition and ordered WHEN ... THEN ... clauses in indented logical-plan output.
• Adds the same information to PostgreSQL JSON output and identifies DML nodes as Dml rather than Projection.
• Adds text and JSON assertions covering aliases, quoted columns, predicates, updates, deletes, and inserts.

This PR is independent of #24195 and targets main directly. The two PRs only modify separate expectations in the shared merge_into.slt test file.

What is the testing strategy for this PR?

• cargo fmt --all
• cargo check -p datafusion-expr -p datafusion-sql
• cargo test -p datafusion-expr write_op_merge_into_name_and_display -- --nocapture
• cargo test -p datafusion-sql plan_merge_into_preserves_target_qualifier_and_quoted_columns -- --nocapture
• cargo test --profile ci --test sqllogictests -- merge_into.slt --nocapture
• cargo clippy --all-targets --all-features -- -D warnings
• Full extended workspace test command from the contributor guide

Are there any user-facing changes?

Yes. EXPLAIN text and PostgreSQL JSON output for MERGE INTO now include the merge condition and clauses. There are no public API signature changes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 76.38889% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.66%. Comparing base (c3ef346) to head (4d15378).

Files with missing lines	Patch %	Lines
datafusion/expr/src/logical_plan/dml.rs	73.58%	2 Missing and 12 partials ⚠️
datafusion/expr/src/logical_plan/plan.rs	60.00%	0 Missing and 2 partials ⚠️
datafusion/expr/src/logical_plan/display.rs	92.85%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #26038   +/-   ##
=======================================
  Coverage   82.66%   82.66%           
=======================================
  Files        1147     1147           
  Lines      446357   446426   +69     
  Branches   446357   446426   +69     
=======================================
+ Hits       368971   369041   +70     
+ Misses      54997    54982   -15     
- Partials    22389    22403   +14     

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[kosiew]

@wirybeaver,

Thanks for improving the MERGE INTO logical-plan display. The change addresses the issue by exposing the ON condition and ordered WHEN clauses in both text and PostgreSQL JSON output. The approach is appropriate, and the additional regression coverage exercises the main formatting paths.

The formatting responsibility stays with the types that own the merge details, while the logical-plan display layers decide how to present them. This preserves the existing planning and execution boundaries, avoids duplicating action formatting, and keeps non-MERGE text output unchanged. The additional complexity is justified by the diagnostic value.

I'm approving because the issue requirements are satisfied and I found no blocking correctness concerns. I left two non-blocking suggestions concerning identifier quoting and stronger JSON contract tests, including coverage of the broader DML node-type change.

[kosiew]

Could we use datafusion_common::utils::quote_identifier for UPDATE assignment targets and INSERT column names when quoting is needed? Currently First Name loses its identifier boundaries, and a name like a, b would look like two columns. Please adjust the existing test and include a comma or embedded-quote identifier. This is a readability improvement, not an execution issue.

[kosiew]

Could we parse the JSON and assert the root Clauses array contains both expected strings in SQL order, along with On, Operation, and Table Name? The substring checks miss the array shape, second clause, and ordering. It would also help to test non-MERGE DML for the changed Node Type and absence of MERGE-only keys, and confirm that the broader node-type change is intentional.