pg_reactive Example 3 -- GROUP BY + SUM

Revenue Dashboard

Aggregate revenue updates in real-time as orders are placed
Disconnected

Revenue by Customer

Customer Revenue Details

ID Customer Region Revenue Orders
This query uses LEFT JOIN + GROUP BY + SUM -- aggregate functions that CDC tools (Debezium, Supabase, ElectricSQL) and Hasura cannot express. pg_reactive recomputes the aggregate on every DML and delivers the exact delta. Customers with zero orders still appear thanks to the LEFT JOIN. 
-- Connect to the database
-- docker compose exec -T db psql -U postgres -d pg_reactive_test

-- Big order for Alice (her bar grows)
INSERT INTO orders (customer_id, product_id, qty, total)
  VALUES (1, 3, 5, 1749.95);

-- First order for Dave (bar appears from zero)
INSERT INTO orders (customer_id, product_id, qty, total)
  VALUES (4, 1, 10, 799.90);

-- New customer appears with $0 (LEFT JOIN)
INSERT INTO customers (name, region)
  VALUES ('Eve', 'eu-west');

-- Delete an order (revenue decreases)
DELETE FROM orders
  WHERE customer_id = 3 AND total = 349.99;