diff --git a/engine/table/src/main/java/io/deephaven/engine/table/impl/select/analyzers/SelectColumnLayer.java b/engine/table/src/main/java/io/deephaven/engine/table/impl/select/analyzers/SelectColumnLayer.java
index 5d60c251e64..cede68b8895 100644
--- a/engine/table/src/main/java/io/deephaven/engine/table/impl/select/analyzers/SelectColumnLayer.java
+++ b/engine/table/src/main/java/io/deephaven/engine/table/impl/select/analyzers/SelectColumnLayer.java
@@ -221,9 +221,7 @@ private void prepareParallelUpdate(final JobScheduler jobScheduler, final TableU
         final int numTasks = splitUpdates.size();
         final long[] destinationOffsets = new long[numTasks];
         if (flattenedResult) {
-            Assert.assertion(upstream.removed().isEmpty(), "upstream.removed().isEmpty()");
-            Assert.assertion(upstream.modified().isEmpty(), "upstream.modified().isEmpty()");
-            Assert.assertion(upstream.shifted().empty(), "upstream.shifted().empty()");
+            // Note that prepareSourcesForParallelPopulation asserts that upstream has no removes, modifies, or shifts
             long destinationOffset = 0;
             for (int ti = 0; ti < numTasks; ++ti) {
                 final TableUpdate splitUpdate = splitUpdates.get(ti);
@@ -552,13 +550,29 @@ private void doEnsureCapacity() {
         }
     }
 
-    void prepareSourcesForParallelPopulation(TableUpdate upstream) {
-        // we do not permit in-column parallelization with redirected results, so do not need to worry about how this
-        // interacts with the previous clearing of the redirection index that has occurred at the start of applyUpdate
-        try (final WritableRowSet changedRows = upstream.added().union(upstream.getModifiedPreShift())) {
-            changedRows.insert(upstream.removed());
-            ((WritableSourceWithPrepareForParallelPopulation) (writableSource))
-                    .prepareForParallelPopulation(changedRows);
+    void prepareSourcesForParallelPopulation(@NotNull final TableUpdate upstream) {
+        // We do not permit in-column parallelization with redirected results, so do not need to worry about how this
+        // interacts with the previous clearing of the RowRedirection that has occurred at the start of applyUpdate.
+        Assert.eqFalse(isRedirected, "isRedirected");
+
+        if (flattenedResult) {
+            // For flattened results the input table must be static; upstream is a "fake" update, and is not permitted
+            // to have removes, modifies, or shifts.
+            Assert.assertion(upstream.removed().isEmpty(), "upstream.removed().isEmpty()");
+            Assert.assertion(upstream.modified().isEmpty(), "upstream.modified().isEmpty()");
+            Assert.assertion(upstream.shifted().empty(), "upstream.shifted().empty()");
+            try (final RowSequence flattenedChanges = RowSequenceFactory.forRange(0, upstream.added().size() - 1)) {
+                ((WritableSourceWithPrepareForParallelPopulation) (writableSource))
+                        .prepareForParallelPopulation(flattenedChanges);
+            }
+        } else {
+            // Upstream is not permitted to have shifts for parallel update processing.
+            Assert.assertion(upstream.shifted().empty(), "upstream.shifted().empty()");
+            try (final WritableRowSet changedRows = upstream.added().union(upstream.modified())) {
+                changedRows.insert(upstream.removed());
+                ((WritableSourceWithPrepareForParallelPopulation) (writableSource))
+                        .prepareForParallelPopulation(changedRows);
+            }
         }
     }