diff --git a/src/dataflow-types/src/types.rs b/src/dataflow-types/src/types.rs
index 81a49f7e7c753..5f26385ab0c89 100644
--- a/src/dataflow-types/src/types.rs
+++ b/src/dataflow-types/src/types.rs
@@ -1438,6 +1438,31 @@ pub mod sources {
             }
         }
 
+        /// Returns `true` if this connector yields data that is
+        /// append-only/monotonic. Append-monly means the source
+        /// never produces retractions.
+        // TODO(guswynn): consider enforcing this more completely at the
+        // parsing/typechecking level, by not using an `envelope`
+        // for sources like pg
+        pub fn append_only(&self) -> bool {
+            match self {
+                // Postgres can produce retractions (deletes)
+                SourceConnector::External {
+                    connector: ExternalSourceConnector::Postgres(_),
+                    ..
+                } => false,
+                // Local sources (i.e., tables) also support retractions (deletes)
+                SourceConnector::Local { .. } => false,
+                // Other sources the `None` envelope are append-only
+                SourceConnector::External {
+                    envelope: SourceEnvelope::None(_),
+                    ..
+                } => true,
+                // Other envelopes can produce retractions
+                _ => false,
+            }
+        }
+
         pub fn name(&self) -> &'static str {
             match self {
                 SourceConnector::External { connector, .. } => connector.name(),
diff --git a/src/transform/src/dataflow.rs b/src/transform/src/dataflow.rs
index 98408a96ec3f0..3a639748033dc 100644
--- a/src/transform/src/dataflow.rs
+++ b/src/transform/src/dataflow.rs
@@ -351,11 +351,7 @@ where
 pub fn optimize_dataflow_monotonic(dataflow: &mut DataflowDesc) -> Result<(), TransformError> {
     let mut monotonic = std::collections::HashSet::new();
     for (source_id, source) in dataflow.source_imports.iter_mut() {
-        if let mz_dataflow_types::sources::SourceConnector::External {
-            envelope: mz_dataflow_types::sources::SourceEnvelope::None(_),
-            ..
-        } = source.description.connector
-        {
+        if source.description.connector.append_only() {
             monotonic.insert(source_id.clone());
         }
     }