PostgreSQL 19 introduces WAIT FOR LSN to solve a real problem in modern apps: when you write data and immediately read it back from a replica, you often get stale data because replication lag is invisible to your application. This feature lets you wait for the replica to catch up before reading, eliminating the guesswork of sleep delays or Redis flags.

• Naive reads from replicas miss 99.2% of the time in the test setup because the app has no way to know if the replica has replayed the write yet—WAIT FOR LSN fixes this by blocking until a specific WAL position is replayed, then succeeds with only 1-2ms overhead.
• Synchronous commit doesn't actually solve this problem; even with remote_apply, you're paying the cost on every write including batch jobs that never read from replicas, whereas WAIT FOR only blocks the specific reads that need consistency.
• The timeout parameter is an architectural decision, not a tuning knob—when lag exceeds your budget, reads fall back to the primary, which can create a stampede during cluster-wide lag events and exhaust connection pools.