[GitHub] [arrow] westonpace commented on a diff in pull request #12852: ARROW-15583: [C++] The Substrait consumer could potentially use a massive amount of RAM if the producer uses large anchors

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

westonpace commented on code in PR #12852:
URL: https://github.com/apache/arrow/pull/12852#discussion_r856373437


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cpp/src/arrow/engine/substrait/extension_set.h:
##########
@@ -226,14 +246,10 @@ class ARROW_ENGINE_EXPORT ExtensionSet {
  private:
   ExtensionIdRegistry* registry_;
   /// The subset of extension registry URIs referenced by this extension set
-  std::vector<util::string_view> uris_;
+  std::unordered_map<uint32_t, util::string_view> uris_;
   std::vector<TypeRecord> types_;
-
   std::vector<FunctionRecord> functions_;
-
-  // pimpl pattern to hide lookup details
-  struct Impl;
-  std::unique_ptr<Impl, void (*)(Impl*)> impl_;
+  std::unordered_map<Id, uint32_t, IdHashEq, IdHashEq> types_map_, functions_map_;

Review Comment:
   Do we have a guideline for hiding.  I find it hard to know when details are "too messy" and deserve a pimpl (other than mutex which we have lint checks for).
   
   Given that users of this header are going to have to dealing with `Id` (`ExtensionIdRegistry` exposes the idea) the `IdHashEq` could be considered useful.



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