[SPARK-54172][SQL] Merge Into Schema Evolution should only add referenced columns #52866
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What changes were proposed in this pull request?
Change MERGE INTO schema evolution scope. Limit the scope of schema evolution to only add columns/nested fields that are referenced in the MERGE INTO query via UPDATE or INSERT statements.
Why are the changes needed?
#51698 added schema evolution support for MERGE INTO statements. However, it is a bit too broad. In some instances, source table may have many more fields than target tables. But user may only need a few new ones to be added to the target for the MERGE INTO statement.
Does this PR introduce any user-facing change?
No, MERGE INTO schema evolution is not yet released in Spark 4.1.
How was this patch tested?
Added many unit tests in MergeIntoTableSuiteBase
Was this patch authored or co-authored using generative AI tooling?
No