rdblue commented on code in PR #9556:
URL: https://github.com/apache/iceberg/pull/9556#discussion_r1468925515
##########
spark/v3.4/spark-extensions/src/main/scala/org/apache/spark/sql/catalyst/analysis/RewriteMergeIntoTable.scala:
##########
@@ -214,6 +214,8 @@ object RewriteMergeIntoTable extends
RewriteRowLevelIcebergCommand with Predicat
val rowFromSourceAttr = resolveAttrRef(ROW_FROM_SOURCE_REF, joinPlan)
val rowFromTargetAttr = resolveAttrRef(ROW_FROM_TARGET_REF, joinPlan)
+ // The output expression should retain read attributes for correctly
determining nullability
+ val matchedOutputsWithAttrs = matchedActions.map(matchedActionOutput(_,
metadataAttrs) :+ readAttrs)
Review Comment:
This doesn't make sense to me. This does have the intended effect to
correctly set the nullability in `buildMergeRowsOutput`, it seems to me like
there is a better way than creating a different (fake) version of
`matchedOutputs`. I think this works because it is including the original
attributes as fake outputs (_for every action output list_). A more direct way
of doing the same thing is to make an attribute nullable if it is nullable OR
if an output is nullable.
In other words, I think the actual problem is in
`RewriteRowLevelIcebergCommand.buildMergingOutput`:
```scala
protected def buildMergingOutput(
outputs: Seq[Seq[Expression]],
attrs: Seq[Attribute]): Seq[Attribute] = {
// build a correct nullability map for output attributes
// an attribute is nullable if at least one output may produce null
val nullabilityMap = attrs.indices.map { index =>
index -> outputs.exists(output => output(index).nullable)
}.toMap
attrs.zipWithIndex.map { case (attr, index) =>
AttributeReference(attr.name, attr.dataType, nullabilityMap(index))()
}
}
```
The change in the current PR updates the `outputs` arg to that method to
include a copy of the original `attrs` for every matched expression. What ends
up happening is that `outputs.exists` checks whether the corresponding original
attr is nullable. You could do the same without this PR's change like this:
```scala
protected def buildMergingOutput(
outputs: Seq[Seq[Expression]],
attrs: Seq[Attribute]): Seq[Attribute] = {
// build a correct nullability map for output attributes
// an attribute is nullable if at least one output may produce null
val nullabilityMap = attrs.indices.map { index =>
index -> outputs.exists(output => output(index).nullable)
}.toMap
attrs.zipWithIndex.map { case (attr, index) =>
AttributeReference(attr.name, attr.dataType, attr.nullable ||
nullabilityMap(index))()
}
}
```
Looking at this code also raises a larger question: what is
`buildMergingOutput` trying to do? It basically copies the input attrs to be
the `MergeRows` output attrs, but then bases nullability on whether unrelated
expressions are nullable. The problem is the expression coming from an output
comes from an `Assignment` and has nothing to do with the position in the input
attrs.
@aokolnychyi do you know what's happening here? Why would the output of
assignment expressions affect the nullability of the incoming data?
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