Read your writes: WAIT FOR in PostgreSQL 19

PostgreSQL 19, currently in beta, adds the `WAIT FOR LSN` command to let sessions wait until write-ahead log progress reaches a specified location, enabling read-your-writes consistency on asynchronous replicas without requiring every primary commit to wait for replica replay. After a write, an application or intermediary can obtain the primary’s current WAL insert LSN and issue `WAIT FOR` on a standby before reading, ensuring the relevant changes have been replayed while avoiding application-side polling. The command supports replay, flush, and write wait modes, optional timeouts, and a `NO_THROW` option that returns statuses such as success, timeout, or not in recovery. It must be executed as a top-level, snapshot-free utility command because holding a snapshot while waiting could block WAL replay and create a self-deadlock. The feature preserves asynchronous write performance by imposing delay only on reads that require freshness, and it may be especially useful for connection poolers or protocol-aware proxies that can automatically track LSNs and route consistent reads to replicas.

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