PostgreSQL subtransactions (nested transactions) may cause multiple performance and scalability issues: higher rates of XID growth and higher risks of transaction ID wraparound, performance degradation when more than PGPROC_MAX_CACHED_SUBXIDS (64) are used in a session, drastic performance slowdowns when subtransactions are combined with SELECT .. FOR UPDATE, and finally, brief downtime on standbys when subtranasctions used on the primary in paralell with long-running transactions or just slo...
