cloud-fan commented on code in PR #35850:
URL: https://github.com/apache/spark/pull/35850#discussion_r877169574
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sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/NestedColumnAliasing.scala:
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@@ -321,6 +321,38 @@ object GeneratorNestedColumnAliasing {
// need to prune nested columns through Project and under Generate. The
difference is
// when `nestedSchemaPruningEnabled` is on, nested columns will be pruned
further at
// file format readers if it is supported.
+
+ // There are [[ExtractValue]] expressions on or not on the output of the
generator. Generator
+ // can also have different types:
+ // 1. For [[ExtractValue]]s not on the output of the generator,
theoretically speaking, there
+ // lots of expressions that we can push down, including non
ExtractValues and GetArrayItem
+ // and GetMapValue. But to be safe, we only handle GetStructField and
GetArrayStructFields.
+ // 2. For [[ExtractValue]]s on the output of the generator, the situation
depends on the type
+ // of the generator expression. *For now, we only support Explode*.
+ // 2.1 Inline
+ // Inline takes an input of ARRAY<STRUCT<field1, field2>>, and
returns an output of
+ // STRUCT<field1, field2>, the output field can be directly accessed
by name "field1".
+ // In this case, we should not try to push down the ExtractValue
expressions to the
+ // input of the Inline. For example:
+ // Project[field1.x AS x]
+ // - Generate[ARRAY<STRUCT<field1: STRUCT<x: int>, field2:int>>,
..., field1, field2]
+ // It is incorrect to push down the .x to the input of the Inline.
+ // A valid field pruning would be to extract all the fields that are
accessed by the
+ // Project, and manually reconstruct an expression using those
fields.
+ // 2.2 Explode
+ // Explode takes an input of ARRAY<some_type> and returns an output
of
+ // STRUCT<col: some_type>. The default field name "col" can be
overwritten.
+ // If the input is MAP<key, value>, it returns STRUCT<key: key_type,
value: value_type>.
+ // For the array case, it is only valid to push down GetStructField.
After push down,
+ // the GetStructField becomes a GetArrayStructFields. Note that we
cannot push down
+ // GetArrayStructFields, since the pushed down expression will
operate on an array of
+ // array which is invalid.
+ // 2.3 Stack
+ // Stack takes a sequence of expressions, and returns an output of
+ // STRUCT<col0: some_type, col1: some_type, ...>
+ // The push down is doable but more complicated in this case as the
expression that
+ // operates on the col_i of the output needs to pushed down to every
(kn+i)-th input
+ // expression where n is the total number of columns (or struct
fields) of the output.
Review Comment:
actually, I find it's useful to understand why we only support explode today.
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