AnishMahto opened a new pull request, #58709:
URL: https://github.com/apache/spark/pull/58709
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### What changes were proposed in this pull request?
Part of the contract for the SCD2 version map is the keys in the version map
should track exactly with the data column names.
Complexity arises when case insensitivity is used, and the incoming
microbatch has a different case-spelling for certain columns than the persisted
target/aux tables.
When `Scd2ForeachBatchHandler` does a [union with the incoming microbatch
rows and the existing target/aux table
rows|https://github.com/apache/spark/blob/master/sql/pipelines/src/main/scala/org/apache/spark/sql/pipelines/autocdc/Scd2ForeachBatchHandler.scala#L104-L106],
due to ordering of the union operators, the resulting DF will respect the
case-spelling on the incoming microbatch.
But when this dataframe is eventually merged back into the target/aux
tables, the existing case-spelling on those tables will win. This is no longer
acceptable since the version map will be constructed by referencing column
names in the microbatch dataframe. If those column names change, the version
map's keys become inconsistent.
The proposal is to union the target schema onto the microbatch dataframe
during microbatch preprocessing, and before we eventually construct the version
map. This appraoch has the added benefit that the version map will now see rows
reductively schema evolved (i.e dropped) from the microbatch.
This change is a no-op behaviorally, and should have no observable side
effect to users.
### Why are the changes needed?
SCD2 ignore-null support
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
Unit tests.
### Was this patch authored or co-authored using generative AI tooling?
Co-authored with GPT-5.6 Sol.
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