[I] Unable to create a list of Maps with Decimal key or value [arrow-java]

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

mdesmet opened a new issue, #69:
URL: https://github.com/apache/arrow-java/issues/69

   ### Describe the usage question you have. Please include as many useful details as  possible.
   
   
   Tested on Arrow 15.0.2
   
   I would like to create a list vector of maps. This works fine for most data types.
   
   However when I'm creating a list of Map<Decimal, Decimal> it fails with following error:
   
   ```
   java.lang.UnsupportedOperationException: Cannot get simple type for type MAP
   
   	at org.apache.arrow.vector.types.Types$MinorType.getType(Types.java:807)
   	at org.apache.arrow.vector.complex.impl.PromotableWriter.getWriter(PromotableWriter.java:274)
   	at org.apache.arrow.vector.complex.impl.AbstractPromotableFieldWriter.getWriter(AbstractPromotableFieldWriter.java:83)
   	at org.apache.arrow.vector.complex.impl.AbstractPromotableFieldWriter.startMap(AbstractPromotableFieldWriter.java:117)
   	at org.apache.arrow.vector.complex.impl.PromotableWriter.startMap(PromotableWriter.java:52)
   	at io.trino.plugin.hive.functions.TestUnloadArrow.testListOfMap(TestUnloadArrow.java:966)
   	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
   	at java.base/java.util.concurrent.RecursiveAction.exec(RecursiveAction.java:194)
   	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:387)
   	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1312)
   	at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1843)
   	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1808)
   	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:188)
   
   ```
   
   ```
       @Test
       public void testListOfMap() {
           try (ListVector from = ListVector.empty("v", new RootAllocator())) {
               UnionListWriter listWriter = from.getWriter();
               listWriter.allocate();
   
               // write null, [null,{"f1":1,"f2":2},null,
               // {"f1":1,"f2":2},null] alternatively
               for (int i = 0; i < 10; i++) {
                   listWriter.setPosition(i);
                   if (i % 2 == 0) {
                       listWriter.writeNull();
                       continue;
                   }
                   listWriter.startList();
                   listWriter.map().startMap();
                   listWriter.map().writeNull();
                   listWriter.map().startEntry();
                   listWriter.map().key();
                   listWriter.map().decimal().writeDecimal(BigDecimal.valueOf(2.0));
                   listWriter.map().value();
                   listWriter.map().decimal().writeDecimal(BigDecimal.valueOf(3.0));
                   listWriter.endEntry();
                   listWriter.map().endMap();
                   listWriter.endList();
               }
               from.setValueCount(10);
   
               System.out.println(from);
           }
       }
   ```
   
   Struct writing works fine, however the Decimal instantation requires scale and precision to be passed in.
   
   Following test works fine.
   
   ```
       @Test
       public void testListOfStruct() {
           try (ListVector from = ListVector.empty("v", new RootAllocator())) {
   
               UnionListWriter listWriter = from.getWriter();
               listWriter.allocate();
   
               // write null, [null,{"f1":1,"f2":2},null,
               // {"f1":1,"f2":2},null] alternatively
               for (int i = 0; i < 10; i++) {
                   listWriter.setPosition(i);
                   if (i % 2 == 0) {
                       listWriter.writeNull();
                       continue;
                   }
                   listWriter.startList();
                   listWriter.struct().writeNull();
                   listWriter.struct().start();
                   listWriter.struct().decimal("f1", 1, 2).writeDecimal(BigDecimal.valueOf(2.0));
                   listWriter.struct().end();
                   listWriter.struct().writeNull();
                   listWriter.struct().start();
                   listWriter.struct().decimal("f1", 1, 2).writeDecimal(BigDecimal.valueOf(3.0));
                   listWriter.struct().end();
                   listWriter.struct().writeNull();
                   listWriter.endList();
               }
               from.setValueCount(10);
   
               System.out.println(from);
           }
       }
   ```
   
   Is there any way to work around this. I have tried to specify the FieldType when creating the ListVector but this fieldType has no child types only the parent type (LIST).
   
   ### Component(s)
   
   Java


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