On 2019-Jan-29, Shreeyansh Dba wrote: > The virtualxid lock is special. It’s a exclusive lock on the transaction’s > own virtual transaction ID that every transaction always holds. No other > transaction can ever acquire it while the transaction is running. > The purpose of this is to allow one transaction to wait until another > transaction commits or rolls back using PostgreSQL’s locking mechanism, and > it’s used internally.
A little more specific than that: it's used when some process (such as CREATE INDEX CONCURRENTLY) needs to wait even on sessions that might be read-only. Such transactions don't have transaction-ids that write transactions have, which is why the only way is to wait on the virtual transaction-id. -- Álvaro Herrera https://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
