Add indexes and pagination to improve app performance

app/routes/data.py

@@ -41,31 +41,36 @@ def manage_data():
 @login_required
 def export_data():
     import io
+    from flask import Response
 
-    output = io.StringIO()
-    writer = csv.writer(output)
+    def generate_csv():
+        """Stream CSV rows to avoid loading all readings into memory."""
+        # Write header
+        output = io.StringIO()
+        writer = csv.writer(output)
+        writer.writerow(['Timestamp', 'Systolic', 'Diastolic', 'Heart Rate'])
+        yield output.getvalue()
+        output.seek(0)
+        output.truncate(0)
 
-    # Write CSV header
-    writer.writerow(['Timestamp', 'Systolic', 'Diastolic', 'Heart Rate'])
+        # Stream readings in chunks using yield_per
+        readings = Reading.query.filter_by(user_id=current_user.id).order_by(
+            Reading.timestamp
+        ).yield_per(500)
 
-    # Write user readings to the CSV
-    readings = Reading.query.filter_by(user_id=current_user.id).all()
-    for reading in readings:
-        writer.writerow([
-            reading.timestamp.strftime('%Y-%m-%d %H:%M:%S'),
-            reading.systolic,
-            reading.diastolic,
-            reading.heart_rate,
-        ])
+        for reading in readings:
+            writer.writerow([
+                reading.timestamp.strftime('%Y-%m-%d %H:%M:%S'),
+                reading.systolic,
+                reading.diastolic,
+                reading.heart_rate,
+            ])
+            yield output.getvalue()
+            output.seek(0)
+            output.truncate(0)
 
-    # Convert text to bytes for `send_file`
-    output.seek(0)
-    response = io.BytesIO(output.getvalue().encode('utf-8'))
-    output.close()
-
-    return send_file(
-        response,
+    return Response(
+        generate_csv(),
         mimetype='text/csv',
-        as_attachment=True,
-        download_name='readings.csv'
+        headers={'Content-Disposition': 'attachment; filename=readings.csv'}
     )