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
