vlsi commented on a change in pull request #2329:
URL: https://github.com/apache/calcite/pull/2329#discussion_r560931469
##########
File path:
linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableDefaults.java
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@@ -4683,4 +4683,191 @@ private void flush() {
}
};
}
+
+ /**
+ * Merge Union Enumerable.
+ * Performs a union (or union all) of all its inputs (which must be already
sorted),
+ * respecting the order.
+ *
+ * @param sources input enumerables (must be already sorted)
+ * @param sortKeySelector sort key selector
+ * @param sortComparator sort comparator to decide the next item
+ * @param all whether duplicates will be considered or not
+ * @param equalityComparer {@link EqualityComparer} to control duplicates,
+ * only used if {@code all} is {@code false}
+ * @param <TSource> record type
+ * @param <TKey> sort key
+ */
+ public static <TSource, @Nullable TKey> Enumerable<TSource> mergeUnion(
+ List<Enumerable<TSource>> sources,
+ Function1<TSource, @Nullable TKey> sortKeySelector,
Review comment:
It would be slightly better to have something like `Function1<? super
TSource, ? extends TKey>` so users could pass method references with subtypes.
I don't know if that combines with other types, however, in most cases
`Function1` arguments should look like `Map#computeIfAbsent`:
```java
public V computeIfAbsent(K key, Function<? super K, ? extends V>
mappingFunction) {
```
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