peter-toth commented on code in PR #56070:
URL: https://github.com/apache/spark/pull/56070#discussion_r3303126793
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sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/SchemaPruning.scala:
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@@ -162,6 +175,80 @@ object SchemaPruning extends SQLConfHelper {
}
}
+ private def getArrayTransformRootField(
+ argument: Expression,
+ function: Expression,
+ elementVar: NamedLambdaVariable): Option[StructField] = {
+ argument.dataType match {
+ case ArrayType(_: StructType, containsNull) =>
+ val selectedFields = collectLambdaVariableFields(function, elementVar)
+ if (selectedFields.exists(_.nonEmpty)) {
+ val mergedElementSchema = selectedFields
+ .get
+ .map(field => StructType(Array(field)))
+ .reduceLeft(_ merge _)
+ SelectedField.withDataType(
+ argument,
+ ArrayType(mergedElementSchema, containsNull))
+ } else {
+ None
+ }
+ case _ => None
+ }
+ }
+
+ /**
+ * Collects statically identifiable nested fields read from `elementVar`.
+ *
+ * `Some(Seq.empty)` means this subtree does not reference the element
variable, and
+ * `Some(fields)` means every reference can be satisfied by the listed
nested fields. `None`
+ * means the full element is required somewhere (for example, `x =>
struct(x.a, x)`), so it is
+ * not safe to prune the element struct.
+ */
+ private def collectLambdaVariableFields(
+ expr: Expression,
+ elementVar: NamedLambdaVariable): Option[Seq[StructField]] = {
+ expr match {
+ case LambdaVariableField(field, variable) if
variable.semanticEquals(elementVar) =>
+ Some(field :: Nil)
+ case variable: NamedLambdaVariable if
variable.semanticEquals(elementVar) =>
+ None
+ case _ =>
+ expr.children.foldLeft(Option(Seq.empty[StructField])) {
+ case (Some(fields), child) =>
+ collectLambdaVariableFields(child, elementVar).map(fields ++ _)
+ case (None, _) => None
+ }
+ }
+ }
+
+ /**
+ * Converts a field access rooted at the lambda element into the single
nested
+ * [[StructField]] shape needed by the input array schema. For example,
+ * `x.company.address` becomes `company: struct<address: ...>`.
+ */
+ private object LambdaVariableField {
Review Comment:
The chain walker here only handles `GetStructField`, but `SelectedField`'s
`selectField` (which this is parallel to) handles `GetArrayStructFields`,
`GetArrayItem`, `ElementAt`, `GetMapValue`, `MapKeys`, and `MapValues` as well.
As written, queries like
```sql
SELECT transform(arr, x -> x.subArr[0].field) FROM t
SELECT transform(arr, x -> element_at(x.subArr, 1).field) FROM t
SELECT transform(arr, x -> x.mapField['k'].field) FROM t
```
will read the full inner type for `subArr` / `mapField` because the chain is
broken at the first non-`GetStructField` node, and
`collectLambdaVariableFields` then falls through to `_` and returns the
lambda-var leaf.
As a follow-up improvement PR, `LambdaVariableField` could mirror
`SelectedField`'s case set (with `selectField` taking the lambda-variable leaf
instead of `Attribute` as terminator). The two extractors would then differ
only in the leaf case and could share a parameterized helper. `array<struct>`
columns nested under another `array<struct>` are common, so closing this gap
covers a non-trivial set of real queries.
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