diff --git a/lib/extensions/postgres_cdc_rls/subscriptions.ex b/lib/extensions/postgres_cdc_rls/subscriptions.ex
index 38746eeb3..ccf63f511 100644
--- a/lib/extensions/postgres_cdc_rls/subscriptions.ex
+++ b/lib/extensions/postgres_cdc_rls/subscriptions.ex
@@ -87,6 +87,7 @@ defmodule Extensions.PostgresCdcRls.Subscriptions do
       from
         sub_tables
         on conflict
+        -- coalesce needed: NULL != NULL in unique constraints; NULL selected_columns means all columns
         (subscription_id, entity, filters, action_filter, coalesce(selected_columns, '{}'))
         do update set
         claims = excluded.claims,
@@ -258,55 +259,77 @@ defmodule Extensions.PostgresCdcRls.Subscriptions do
   @spec parse_subscription_params(map()) :: {:ok, subscription_params} | {:error, binary()}
   def parse_subscription_params(params) do
     action_filter = action_filter(params)
-    selected_columns = parse_select(params)
-
-    case params do
-      %{"schema" => schema, "table" => table, "filter" => filter}
-      when is_binary(schema) and is_binary(table) and is_binary(filter) ->
-        case parse_filters(filter) do
-          {:ok, filters} -> {:ok, {action_filter, schema, table, filters, selected_columns}}
-          {:error, reason} -> {:error, "Error parsing `filter` params: #{reason}"}
-        end
 
-      %{"schema" => schema, "table" => table}
-      when is_binary(schema) and is_binary(table) and not is_map_key(params, "filter") ->
-        {:ok, {action_filter, schema, table, [], selected_columns}}
+    with {:ok, selected_columns} <- parse_select(params) do
+      case params do
+        %{"schema" => schema, "table" => table, "filter" => filter}
+        when is_binary(schema) and is_binary(table) and is_binary(filter) ->
+          case parse_filters(filter) do
+            {:ok, filters} ->
+              case reject_select_on_wildcard(schema, table, selected_columns) do
+                :ok -> {:ok, {action_filter, schema, table, filters, selected_columns}}
+                error -> error
+              end
+
+            {:error, reason} ->
+              {:error, "Error parsing `filter` params: #{reason}"}
+          end
+
+        %{"schema" => schema, "table" => table}
+        when is_binary(schema) and is_binary(table) and not is_map_key(params, "filter") ->
+          case reject_select_on_wildcard(schema, table, selected_columns) do
+            :ok -> {:ok, {action_filter, schema, table, [], selected_columns}}
+            error -> error
+          end
 
-      %{"schema" => schema}
-      when is_binary(schema) and not is_map_key(params, "table") and
-             not is_map_key(params, "filter") ->
-        {:ok, {action_filter, schema, "*", [], selected_columns}}
+        %{"schema" => schema}
+        when is_binary(schema) and not is_map_key(params, "table") and
+               not is_map_key(params, "filter") ->
+          case reject_select_on_wildcard(schema, "*", selected_columns) do
+            :ok -> {:ok, {action_filter, schema, "*", [], selected_columns}}
+            error -> error
+          end
 
-      %{"table" => table}
-      when is_binary(table) and not is_map_key(params, "schema") and
-             not is_map_key(params, "filter") ->
-        {:ok, {action_filter, "public", table, [], selected_columns}}
+        %{"table" => table}
+        when is_binary(table) and not is_map_key(params, "schema") and
+               not is_map_key(params, "filter") ->
+          case reject_select_on_wildcard("public", table, selected_columns) do
+            :ok -> {:ok, {action_filter, "public", table, [], selected_columns}}
+            error -> error
+          end
 
-      map when is_map_key(map, "user_token") or is_map_key(map, "auth_token") ->
-        {:error,
-         "No subscription params provided. Please provide at least a `schema` or `table` to subscribe to: <redacted>"}
+        map when is_map_key(map, "user_token") or is_map_key(map, "auth_token") ->
+          {:error,
+           "No subscription params provided. Please provide at least a `schema` or `table` to subscribe to: <redacted>"}
 
-      error ->
-        {:error,
-         "No subscription params provided. Please provide at least a `schema` or `table` to subscribe to: #{inspect(error)}"}
+        error ->
+          {:error,
+           "No subscription params provided. Please provide at least a `schema` or `table` to subscribe to: #{inspect(error)}"}
+      end
     end
   end
 
   defp parse_select(%{"select" => cols}) when is_list(cols) do
     case Enum.filter(cols, &is_binary/1) do
-      [] -> nil
-      valid -> valid
+      [] -> {:ok, nil}
+      valid -> {:ok, valid}
     end
   end
 
   defp parse_select(%{"select" => str}) when is_binary(str) do
-    case str |> String.split(",") |> Enum.reject(&(&1 == "")) do
-      [] -> nil
-      cols -> cols
-    end
+    {:error, "Error parsing `select` params: expected a list of column name strings, e.g. select: [\"col1\", \"col2\"]"}
+  end
+
+  defp parse_select(_), do: {:ok, nil}
+
+  defp reject_select_on_wildcard(_schema, _table, nil), do: :ok
+
+  defp reject_select_on_wildcard(schema, table, _selected_columns)
+       when schema == "*" or table == "*" do
+    {:error, "Column selection is not supported for wildcard subscriptions. Provide an explicit schema and table name."}
   end
 
-  defp parse_select(_), do: nil
+  defp reject_select_on_wildcard(_schema, _table, _selected_columns), do: :ok
 
   defp action_filter(%{"event" => "*"}), do: "*"
 
diff --git a/test/realtime/extensions/cdc_rls/subscriptions_test.exs b/test/realtime/extensions/cdc_rls/subscriptions_test.exs
index f32235d5b..43a77fad4 100644
--- a/test/realtime/extensions/cdc_rls/subscriptions_test.exs
+++ b/test/realtime/extensions/cdc_rls/subscriptions_test.exs
@@ -619,31 +619,15 @@ defmodule Realtime.Extensions.PostgresCdcRls.SubscriptionsTest do
                })
     end
 
-    test "user can pass a comma-separated string of column names as an alternative syntax" do
-      assert {:ok, {"*", "public", "messages", [], ["id", "details"]}} =
+    test "passing a string to select is rejected with a clear error message" do
+      assert {:error, msg} =
                Subscriptions.parse_subscription_params(%{
                  "schema" => "public",
                  "table" => "messages",
                  "select" => "id,details"
                })
-    end
-
-    test "consecutive commas in the select string are treated as empty segments and ignored" do
-      assert {:ok, {"*", "public", "messages", [], ["id", "details"]}} =
-               Subscriptions.parse_subscription_params(%{
-                 "schema" => "public",
-                 "table" => "messages",
-                 "select" => "id,,details"
-               })
-    end
 
-    test "column names with surrounding spaces are not trimmed and will be rejected by the database" do
-      assert {:ok, {"*", "public", "messages", [], ["id", " details"]}} =
-               Subscriptions.parse_subscription_params(%{
-                 "schema" => "public",
-                 "table" => "messages",
-                 "select" => "id, details"
-               })
+      assert msg =~ "`select`"
     end
 
     test "non-binary entries in a select list are silently dropped" do
@@ -655,22 +639,15 @@ defmodule Realtime.Extensions.PostgresCdcRls.SubscriptionsTest do
                })
     end
 
-    test "passing an empty select string is treated as no column selection" do
-      assert {:ok, {"*", "public", "messages", [], nil}} =
+    test "passing any string value to select is rejected" do
+      assert {:error, msg} =
                Subscriptions.parse_subscription_params(%{
                  "schema" => "public",
                  "table" => "messages",
                  "select" => ""
                })
-    end
 
-    test "passing a bare comma as the select string is treated as no column selection" do
-      assert {:ok, {"*", "public", "messages", [], nil}} =
-               Subscriptions.parse_subscription_params(%{
-                 "schema" => "public",
-                 "table" => "messages",
-                 "select" => ","
-               })
+      assert msg =~ "`select`"
     end
 
     test "user can combine column selection with a row filter" do
@@ -753,6 +730,27 @@ defmodule Realtime.Extensions.PostgresCdcRls.SubscriptionsTest do
 
       assert msg =~ "invalid column for select nonexistent_column"
     end
+
+    test "user gets an error when using select with a schema-only (wildcard table) subscription" do
+      assert {:error, msg} =
+               Subscriptions.parse_subscription_params(%{
+                 "schema" => "public",
+                 "select" => ["id"]
+               })
+
+      assert msg =~ "wildcard"
+    end
+
+    test "user gets an error when using select with an explicit wildcard table" do
+      assert {:error, msg} =
+               Subscriptions.parse_subscription_params(%{
+                 "schema" => "public",
+                 "table" => "*",
+                 "select" => ["id"]
+               })
+
+      assert msg =~ "wildcard"
+    end
   end
 
   defp create_subscriptions(conn, num) do