Re: [I] [C++][Acero] Support join matching missing value [arrow]

["zanmato1984 (via GitHub)" <gi...@apache.org>]

zanmato1984 commented on issue #41358:
URL: https://github.com/apache/arrow/issues/41358#issuecomment-2075508642

   Acero does support this kind of join, which is enabled by `JoinKeyCmp::IS`:
   https://github.com/apache/arrow/blob/fd75cbdb38836b66dbae81094cf06821e5d25cb1/cpp/src/arrow/acero/options.h#L558
   
   So in C++, you are well able to do this as shown in the following quick demo:
   ```
   TEST(HashJoin, JoinKeyCmp) {
     std::vector<TypeHolder> left_types = {utf8()};
     auto left_batch = ExecBatchFromJSON(left_types, R"([["a"], [null], ["b"]])");
     std::vector<TypeHolder> right_types = {utf8(), int32()};
     auto right_batch = ExecBatchFromJSON(right_types, R"([["a", 1], [null, 2], ["b", 3]])");
   
     Declaration left{
         "exec_batch_source",
         ExecBatchSourceNodeOptions(schema({field("l_key", utf8())}), {left_batch})};
     Declaration right{
         "exec_batch_source",
         ExecBatchSourceNodeOptions(
             schema({field("r_key", utf8()), field("r_value", int32())}), {right_batch})};
   
     {
       std::cout << "Joining using JoinKeyCmp::EQ" << std::endl;
       HashJoinNodeOptions join_opts(JoinType::LEFT_OUTER, /*left_keys=*/{"l_key"},
                                     /*right_keys*/ {"r_key"}, /*left_output=*/{"l_key"},
                                     /*right_output=*/{"r_key", "r_value"},
                                     /*NOTE: key_cmp=*/{JoinKeyCmp::EQ});
       Declaration join{"hashjoin", {left, right}, join_opts};
       ASSERT_OK_AND_ASSIGN(auto result, DeclarationToTable(std::move(join)));
       std::cout << result->ToString() << std::endl;
     }
   
     {
       std::cout << "Joining using JoinKeyCmp::IS" << std::endl;
       HashJoinNodeOptions join_opts(JoinType::LEFT_OUTER, /*left_keys=*/{"l_key"},
                                     /*right_keys*/ {"r_key"}, /*left_output=*/{"l_key"},
                                     /*right_output=*/{"r_key", "r_value"},
                                     /*NOTE: key_cmp=*/{JoinKeyCmp::IS});
       Declaration join{"hashjoin", {left, right}, join_opts};
       ASSERT_OK_AND_ASSIGN(auto result, DeclarationToTable(std::move(join)));
       std::cout << result->ToString() << std::endl;
     }
   }
   ```
   
   Output:
   ```
   Joining using JoinKeyCmp::EQ
   ----
   l_key:
     [
       [
         "a",
         "b",
         null
       ]
     ]
   r_key:
     [
       [
         "a",
         "b",
         null
       ]
     ]
   r_value:
     [
       [
         1,
         3,
         null
       ]
     ]
   
   Joining using JoinKeyCmp::IS
   ----
   l_key:
     [
       [
         "a",
         null,
         "b"
       ]
     ]
   r_key:
     [
       [
         "a",
         null,
         "b"
       ]
     ]
   r_value:
     [
       [
         1,
         2,
         3
       ]
     ]
   ```
   
   But I'm not familiar with R binding so I can't really tell if this is available in R.


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