fix(scd2): preserve NULL values in historical rows - #6115
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Select target values using an explicit row-existence flag carried into the SCD2 join. Choose a flag alias that does not overlap source or prefixed target columns, including case-insensitive names. Co-authored-by: Nicholas <71755971+ionnich@users.noreply.github.com> Signed-off-by: Arthur031221 <levi74108520963@gmail.com>
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Description
Users running SCD_TYPE_2_BY_TIME or SCD_TYPE_2_BY_COLUMN models lost historical NULL values when a later source row supplied a value for the same key.
updated_rowsusedCOALESCE(target, source), so a NULL target fell through to the new source value. It now uses the target row's existence flag to keep its values, including NULL. The existence check also preserves target payloads when a key or validity timestamp is NULL. Its alias is chosen to avoid source and prefixed target column names.This builds on the code @ionnich posted in issue #5332. Fixes #5332.
Test Plan
make fast-test,make style,make doc-test, andmake benchmark-cipassed on Linux with Python 3.12.3. The four history regression cases cover both SCD2 kinds with SQL and Pandas inputs. They fail against upstream production source and pass with the fix. Four additional cases verify that removing a source key preserves the key and payload of an existing target row whose validity timestamp is NULL. Sixteen additional cases cover TEXT and BOOLEAN columns that collide with internal flag names, including case-insensitive names. The collision cases pass against upstream, fail with a constant flag name, and pass with the fix. All 24 added SCD2 regression cases pass with the fix.Checklist
make styleand fixed any issuesmake fast-test)git commit -s) per the DCOCo-authored-by: Nicholas 71755971+ionnich@users.noreply.github.com