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Rather than consulting TransactionIdIsInProgress to see if an in-doubt transaction is still running, consult XidInMVCCSnapshot. That requires the same or fewer cycles as TransactionIdIsInProgress, and what's far more important, it does not access shared data structures (at least in the no-subxip-overflow case) so it incurs no contention. Furthermore, we would have had to check XidInMVCCSnapshot anyway before deciding that we were allowed to see the tuple. There should never be a case where XidInMVCCSnapshot says a transaction is done while TransactionIdIsInProgress says it's still running. The other way around is quite possible though. The result of that difference is that HeapTupleSatisfiesMVCC will no longer set hint bits on tuples whose source transactions recently finished but are still running according to our snapshot. The main cost of delaying the hint-bit setting is that repeated visits to a just-committed tuple, by transactions none of which have snapshots new enough to see the source transaction as done, will each execute TransactionIdIsCurrentTransactionId, which they need not have done before. However, that's normally just a small overhead, and no contention costs are involved; so it seems well worth the benefit of removing TransactionIdIsInProgress calls during the life of the source transaction. The core idea for this patch is due to Jeff Janes, who also did the legwork proving its performance benefits. His original proposal was to swap the order of TransactionIdIsInProgress and XidInMVCCSnapshot calls in some cases within HeapTupleSatisfiesMVCC. That was a bit messy though. The idea that we could dispense with calling TransactionIdIsInProgress altogether was mine, as is the final patch. |
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PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: http://www.postgresql.org/download See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.