PostgreSQL is an open-source object-relational database system that ensures data integrity and reliability with features like Multi-Version Concurrency Control (MVCC) and write-ahead logging. It has a query planner/optimizer that determines the most efficient way to execute queries, accounting for factors such as index usage and internal statistics about the database. The database uses MVCC to ensure that concurrent transactions do not block each other, and it periodically runs checkpoint processes to flush dirty pages from memory to disk. PostgreSQL maintains data reliability by logging each transaction in the write-ahead log (WAL) on the primary and writing it to disk periodically. It also collects metrics about its own resource usage, including connections, shared buffer usage, and disk utilization. The database's performance can be monitored through various key metrics, such as read query throughput and performance, write query throughput and performance, replication and reliability, and resource utilization. These metrics are accessible through PostgreSQL's statistics collector and other native sources, and they provide insights into the health and availability of the database.