The problem

An invoicing product serves 3 000 companies from one PostgreSQL database. Most are bakeries and small agencies with a few hundred invoices a month. One is a retail chain with 300 GB of data.

On the last day of each month the chain runs its reports, and the bakeries' invoice pages take 4 seconds to open.

Last week a developer shipped a query without WHERE tenant_id = .... A customer saw another company's invoice list for 20 minutes.