Need help understanding pg_locks

Can someone help me understand pg_locks? There are three fields related
to virtual and real xids:

virtualtransaction | text |
transactionid | xid |
virtualxid | text |

Our docs say 'virtualtransaction' is:

Virtual ID of the transaction that is holding or awaiting this lock

This field was clear to me.

and 'transactionid' is documented as:

ID of a transaction, or null if the object is not a transaction ID

In my testing it was the (non-virtual) xid of the lock holder. Is that
correct? Can it be a waiter?

'virtualxid' is documented as:

Virtual ID of a transaction, or null if the object is not a
virtual transaction ID

In my testing this field is for locking your own vxid, meaning it owned
by its own vxid.

I looked at the C code in /pg/backend/utils/adt/lockfuncs.c and was
confused.

Clearly our documentation is lacking in this area and I would like to
clarify it.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +