diff --git a/sdks/java/core/src/main/java/org/apache/beam/sdk/testing/PAssert.java b/sdks/java/core/src/main/java/org/apache/beam/sdk/testing/PAssert.java
index e80205a75b305..29defb49a5e59 100644
--- a/sdks/java/core/src/main/java/org/apache/beam/sdk/testing/PAssert.java
+++ b/sdks/java/core/src/main/java/org/apache/beam/sdk/testing/PAssert.java
@@ -50,6 +50,7 @@
 import org.apache.beam.sdk.transforms.MapElements;
 import org.apache.beam.sdk.transforms.PTransform;
 import org.apache.beam.sdk.transforms.ParDo;
+import org.apache.beam.sdk.transforms.Reify;
 import org.apache.beam.sdk.transforms.SerializableFunction;
 import org.apache.beam.sdk.transforms.SimpleFunction;
 import org.apache.beam.sdk.transforms.Values;
@@ -73,6 +74,7 @@
 import org.apache.beam.sdk.values.ValueInSingleWindow;
 import org.apache.beam.sdk.values.WindowingStrategy;
 import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.base.Objects;
+import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.ImmutableList;
 import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.Iterables;
 import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.Lists;
 import org.joda.time.Duration;
@@ -1153,7 +1155,8 @@ public Void apply(T actual) {
   /**
    * A transform that gathers the contents of a {@link PCollection} into a single main input
    * iterable in the global window. This requires a runner to support {@link GroupByKey} in the
-   * global window, but not side inputs or other windowing or triggers.
+   * global window, but not side inputs or other windowing or triggers unless the input is
+   * non-trivially windowed or triggered.
    *
    * <p>If the {@link PCollection} is empty, this transform returns a {@link PCollection} containing
    * a single empty iterable, even though in practice most runners will not produce any element.
@@ -1171,6 +1174,41 @@ public GroupGlobally(AssertionWindows rewindowingStrategy) {
     public PCollection<Iterable<ValueInSingleWindow<T>>> expand(PCollection<T> input) {
       final int combinedKey = 42;
 
+      if (input.getWindowingStrategy().equals(WindowingStrategy.globalDefault())
+          && rewindowingStrategy instanceof IntoGlobalWindow) {
+        // If we don't have to worry about complicated triggering semantics we can generate
+        // a much simpler pipeline.  This is particularly useful for bootstrapping runners so that
+        // we can run subsets of the validates runner test suite requiring support of only the
+        // most basic primitives.
+
+        // In order to ensure we actually get an (empty) iterable rather than an empty PCollection
+        // when the input is an empty PCollection, we flatten with a dummy PCollection containing
+        // an empty iterable before grouping on a singleton key and concatenating.
+        PCollection<Iterable<ValueInSingleWindow<T>>> actual =
+            input.apply(Reify.windows()).apply(ParDo.of(new ToSingletonIterables<>()));
+        PCollection<Iterable<ValueInSingleWindow<T>>> dummy =
+            input
+                .getPipeline()
+                .apply(
+                    Create.<Iterable<ValueInSingleWindow<T>>>of(
+                            ImmutableList.of(ImmutableList.of()))
+                        .withCoder(actual.getCoder()));
+        return PCollectionList.of(dummy)
+            .and(actual)
+            .apply(Flatten.pCollections())
+            .apply(
+                // Default end-of-window trigger disallowed for unbounded PCollections.
+                input.isBounded() == PCollection.IsBounded.UNBOUNDED
+                    ? Window.<Iterable<ValueInSingleWindow<T>>>configure()
+                        .triggering(Never.ever())
+                        .discardingFiredPanes()
+                    : Window.<Iterable<ValueInSingleWindow<T>>>configure())
+            .apply(WithKeys.of(combinedKey))
+            .apply(GroupByKey.create())
+            .apply(Values.create())
+            .apply(ParDo.of(new ConcatFn<>()));
+      }
+
       // Remove the triggering on both
       PTransform<
               PCollection<KV<Integer, Iterable<ValueInSingleWindow<T>>>>,
@@ -1223,9 +1261,16 @@ public PCollection<Iterable<ValueInSingleWindow<T>>> expand(PCollection<T> input
     }
   }
 
+  private static final class ToSingletonIterables<T> extends DoFn<T, Iterable<T>> {
+    @ProcessElement
+    public void processElement(ProcessContext c) {
+      c.output(ImmutableList.of(c.element()));
+    }
+  }
+
   private static final class ConcatFn<T> extends DoFn<Iterable<Iterable<T>>, Iterable<T>> {
     @ProcessElement
-    public void processElement(ProcessContext c) throws Exception {
+    public void processElement(ProcessContext c) {
       c.output(Iterables.concat(c.element()));
     }
   }