[GitHub] [calcite] vlsi commented on a change in pull request #2413: [CALCITE-4603] Least restrictive for collections of collections

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

vlsi commented on a change in pull request #2413:
URL: https://github.com/apache/calcite/pull/2413#discussion_r631740972



##########
File path: core/src/main/java/org/apache/calcite/rel/type/RelDataTypeFactoryImpl.java
##########
@@ -257,6 +260,35 @@ private RelDataType createStructType(
     return createTypeWithNullability(builder.build(), isNullable);
   }
 
+  protected @Nullable RelDataType leastRestrictiveCollectionType(
+      final List<RelDataType> types, SqlTypeName sqlTypeName) {
+    assert sqlTypeName == SqlTypeName.ARRAY
+        || sqlTypeName == SqlTypeName.MULTISET || sqlTypeName == SqlTypeName.MAP;
+    boolean isNullable = false;
+    for (RelDataType type : types) {
+      isNullable |= type.isNullable();
+    }
+    if (sqlTypeName == SqlTypeName.MAP) {
+      RelDataType keyType = leastRestrictive(
+          Util.transform(types, t -> Objects.requireNonNull(t.getKeyType())));

Review comment:
       The reason it complains is that `RelDataType#getKeyType()` is nullable, while `MapSqlType#getKeyType()` is non-nullable.
   The checker is not sure if `getKeyType()` is guaranteed to return the same value every time. 
   
   If you cast `t` to `MapSqlType`, then it would work without `requireNonNull`.
   An alternative approaches are:
   a) Make `leastRestrictive` return `null` when `null` is among its arguments. Currently `leastRestrictive` would fail with NPE
   b) Rework `leastRestrictive` into builder-like API, so it could indicate to the caller that no more types are needed. The user could instantiate a couple of builders (e.g. key builder, value builder), iterate over the types, and build the common type (e.g. right inside `for (RelDataType type: types)`)




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