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Re: What happens to transactions durring a pg_dump?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Bruno Wolff III <bruno(at)wolff(dot)to>
Cc: Jaime Casanova <systemguards(at)gmail(dot)com>, Chris Hoover <revoohc(at)gmail(dot)com>, Michael Fuhr <mike(at)fuhr(dot)org>, "pgsql-admin(at)postgresql(dot)org" <pgsql-admin(at)postgresql(dot)org>
Subject: Re: What happens to transactions durring a pg_dump?
Date: 2006-01-22 00:50:42
Message-ID: 2754.1137891042@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-admin
Bruno Wolff III <bruno(at)wolff(dot)to> writes:
> I don't think that is precisely correct. I believe that the snapshot isn't
> set until the first query is made within the transaction.

Right.  Specifically, any statement *other* than those enumerated in
PortalRunUtility() will set the snapshot.  As of CVS tip (and for
several past releases IIRC), that code looks like:

    /*
     * Set snapshot if utility stmt needs one.    Most reliable way to do this
     * seems to be to enumerate those that do not need one; this is a short
     * list.  Transaction control, LOCK, and SET must *not* set a snapshot
     * since they need to be executable at the start of a serializable
     * transaction without freezing a snapshot.  By extension we allow SHOW
     * not to set a snapshot.  The other stmts listed are just efficiency
     * hacks.  Beware of listing anything that can modify the database --- if,
     * say, it has to update an index with expressions that invoke
     * user-defined functions, then it had better have a snapshot.
     */
    if (!(IsA(utilityStmt, TransactionStmt) ||
          IsA(utilityStmt, LockStmt) ||
          IsA(utilityStmt, VariableSetStmt) ||
          IsA(utilityStmt, VariableShowStmt) ||
          IsA(utilityStmt, VariableResetStmt) ||
          IsA(utilityStmt, ConstraintsSetStmt) ||
    /* efficiency hacks from here down */
          IsA(utilityStmt, FetchStmt) ||
          IsA(utilityStmt, ListenStmt) ||
          IsA(utilityStmt, NotifyStmt) ||
          IsA(utilityStmt, UnlistenStmt) ||
          IsA(utilityStmt, CheckPointStmt)))
        ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
    else
        ActiveSnapshot = NULL;


			regards, tom lane

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