check for start and end existence

ui/app/mirrors/create/handlers.ts

@@ -211,6 +211,16 @@ export const handleCreateQRep = async (
     return;
   }
 
+  if (
+    config.writeMode?.writeType != QRepWriteType.QREP_WRITE_MODE_OVERWRITE &&
+    !(query.includes('{{.start}}') && query.includes('{{.end}}'))
+  ) {
+    notifyErr(
+      'Please include placeholders {{.start}} and {{.end}} in the query'
+    );
+    return;
+  }
+
   if (xmin == true) {
     config.watermarkColumn = 'xmin';
     config.query = `SELECT * FROM ${quotedWatermarkTable(

ui/app/mirrors/create/page.tsx

@@ -191,6 +191,11 @@ export default function CreateMirrors() {
                 Write a query whose results will be replicated to a target
                 table.
                 <br></br>
+                For append and upsert modes, make sure the query{' '}
+                <b>includes the start and end placeholders</b> in the query.
+                PeerDB uses these placeholders for partitioning query results
+                for performance.
+                <br></br>
                 In most cases, you will require a watermark table and a
                 watermark column in that table.
                 <br></br>