SLOW QUERY DEBUGGING WHAT BROKE OUR SAAS APP

Summary from the TLDR Web Dev newsletter

A slow database query calculating user credit balances took down Placid.app when high-volume API requests triggered thousands of concurrent executions of an expensive query that should have been cached. Multiple workers tried to calculate the same user's balance simultaneously and the load ballooned. This caused a cascade of failures, timeouts, and blocked queries that eventually froze the system.