[GitHub] [arrow] pitrou commented on a diff in pull request #13528: ARROW-16989: [C++] Substrait ProjectRel is interpreted incorrectly

[GitBox <gi...@apache.org>]

pitrou commented on code in PR #13528:
URL: https://github.com/apache/arrow/pull/13528#discussion_r917912040


##########
cpp/src/arrow/engine/substrait/relation_internal.cc:
##########
@@ -215,16 +220,24 @@ Result<compute::Declaration> FromProto(const substrait::Rel& rel,
       }
       ARROW_ASSIGN_OR_RAISE(auto input, FromProto(project.input(), ext_set));
 
+      // NOTE: Substrait ProjectRels *append* columns, while Acero's project node replaces
+      // them. Therefore, we need to prefix all the current columns for compatibility.
       std::vector<compute::Expression> expressions;

Review Comment:
   May want to presize this?



##########
cpp/src/arrow/engine/substrait/relation_internal.h:
##########
@@ -30,8 +30,17 @@
 namespace arrow {
 namespace engine {
 
+/// Information resulting from converting a Substrait relation.
+struct DeclarationInfo {
+  /// The compute declaration produced thus far.
+  compute::Declaration declaration;
+
+  /// The number of columns returned by the declaration.
+  size_t num_columns;

Review Comment:
   Right, this can be an `int`.



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