Date: Tue, 29 Jan 2019 12:12:18 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Shreeyansh Dba <shreeyansh2014@gmail.com>
Cc: Mariel Cherkassky <mariel.cherkassky@gmail.com>,
	Pgsql-admin <pgsql-admin@lists.postgresql.org>,
	PostgreSQL mailing lists <pgsql-performance@postgresql.org>
Subject: Re: pg_locks - what is a virtualxid locktype
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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