[GitHub] [arrow] westonpace commented on a diff in pull request #35249: GH-35247: [C++] Add Arrow Substrait support for stddev/variance

["westonpace (via GitHub)" <gi...@apache.org>]

westonpace commented on code in PR #35249:
URL: https://github.com/apache/arrow/pull/35249#discussion_r1173952572


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cpp/src/arrow/engine/substrait/extension_set.cc:
##########
@@ -944,6 +944,30 @@ ExtensionIdRegistry::SubstraitAggregateToArrow DecodeBasicAggregate(
                                call.id().name, " to have at least one argument");
       }
       case 1: {
+        std::shared_ptr<compute::FunctionOptions> options = nullptr;
+        if (arrow_function_name == "stddev" || arrow_function_name == "variance") {
+          // See the following URL for the spec of stddev and variance:
+          // https://github.com/substrait-io/substrait/blob/
+          // 73228b4112d79eb1011af0ebb41753ce23ca180c/
+          // extensions/functions_arithmetic.yaml#L1240
+          auto maybe_dist = call.GetOption("distribution");
+          if (maybe_dist) {
+            auto& prefs = **maybe_dist;
+            if (prefs.size() != 1) {
+              return Status::Invalid("expected a single preference for ",
+                                     arrow_function_name, " but got ", prefs.size());
+            }

Review Comment:
   Technically multiple preferences means the user would prefer the first value but will be ok with the second.  In other words, it's similar to https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept
   
   So you should be able to handle this case.  However, no producers do this yet.



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