Proposal: Snapshot cloning

Granted this one has a few open ends so far and I'd like to receive some 
constructive input on how to actually implement it.

The idea is to clone an existing serializable transactions snapshot 
visibility information from one backend to another. The semantics would 
be like this:
    backend1: start transaction;    backend1: set transaction isolation level serializable;    backend1: select
pg_backend_pid();   backend1: select publish_snapshot(); -- will block
 
    backend2: start transaction;    backend2: set transaction isolation level serializable;    backend2: select
clone_snapshot(<pid>);-- will unblock backend1
 
    backend1: select publish_snapshot();
    backend3: start transaction;    backend3: set transaction isolation level serializable;    backend3: select
clone_snapshot(<pid>);
    ...

This will allow a number of separate backends to assume the same MVCC 
visibility, so that they can query independent but the overall result 
will be according to one consistent snapshot of the database.

What I try to accomplish with this is to widen a bottleneck, many 
current Slony users are facing. The initial copy of a database is 
currently limited to one single reader to copy a snapshot of the data 
provider. With the above functionality, several tables could be copied 
in parallel by different client threads, feeding separate backends on 
the receiving side at the same time.

The feature could also be used by a parallel version of pg_dump as well 
as data mining tools.

The cloning process needs to make sure that the clone_snapshot() call is 
made from the same DB user in the same database as corresponding 
publish_snapshot() call was done. Since publish_snapshot() only 
publishes the information, it gained legally and that is visible in the 
PGPROC shared memory (xmin, xmax being the crucial part here), there is 
no risk of creating a snapshot for which data might have been removed by 
vacuum already.

What I am not sure about yet is what IPC method would best suit the 
transfer of the arbitrarily sized xip vector. Ideas?


Jan

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