[SPARK] Support CTAS from a table with a deleted default value #4142
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c27kwan
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Feb 11, 2025
spark/src/test/scala/org/apache/spark/sql/delta/DeltaTableCreationTests.scala
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spark/src/test/scala/org/apache/spark/sql/delta/DeltaTableCreationTests.scala
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spark/src/main/scala/org/apache/spark/sql/delta/commands/CreateDeltaTableCommand.scala
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spark/src/main/scala/org/apache/spark/sql/delta/commands/CreateDeltaTableCommand.scala
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cstavr
approved these changes
Feb 25, 2025
spark/src/main/scala/org/apache/spark/sql/delta/OptimisticTransaction.scala
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spark/src/main/scala/org/apache/spark/sql/delta/sources/DeltaSQLConf.scala
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scottsand-db
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Feb 26, 2025
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Which Delta project/connector is this regarding?
Description
We are currently leaking values using the 'EXISTS_DEFAULT' config key that is used for default columns in Spark into table schemas. The 'EXISTS_DEFAULT' key holds values for rows that existed when a new column with a default value was added to the table. This use case is not supported in Delta.
In this PR, we remove the 'EXISTS_DEFAULT' entries every time we update the metadata of a table. This includes the case when a table is created. This ensures that new tables do not have 'EXISTS_DEFAULT' entries and operations that update metadata remove entries that existed before because previous versions added them.
This change also now allows us to CTAS from a table where a default value existed in the past but has been dropped. It does not enable CTAS from a table with an active default value.
The feature has a kill switch: REMOVE_EXISTS_DEFAULT_FROM_SCHEMA_ON_EVERY_METADATA_CHANGE
How was this patch tested?
Added new tests
Does this PR introduce any user-facing changes?
Previously it was not possible to CTAS from a table with a removed default without setting the delta.feature.allowColumnDefaults even though no default value was actually part of the new table. this is now possible.