shehab-ali commented on PR #21262:
URL: https://github.com/apache/datafusion/pull/21262#issuecomment-4268903339
Problem: When SessionState is rebuilt via
SessionStateBuilder::new_from_existing(), a user's UDF override can be silently
reverted, causing the wrong function to execute at query time.
Concrete example: a user registers postgres_to_char with alias "to_char" to
override the built-in to_char. Queries using to_char() correctly call the
custom implementation. But when any code path rebuilds the session state,
HashMap::into_values() emits both the custom UDF and the built-in (which
survived under its "date_format" alias key). build() re-registers both, and
last writer wins. If the built-in happens to register last, "to_char" now
points back to the built-in, and subsequent queries silently call the wrong
function. This happens nondeterministically depending on HashMap iteration
order.
What changed: the filter key.as_str() == udf.name() only keeps entries where
the HashMap key is the UDF's canonical name. This drops the built-in because
its canonical key ("to_char") was overwritten by the custom UDF; it only exists
under "date_format", which doesn't match its canonical name "to_char". So the
built-in is never passed to build(), and the custom UDF's override is preserved.
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