ae632f7a31
Commit bdaabb9b started skipping doomed transactions when building the list of possible conflicts for SERIALIZABLE READ ONLY. That makes sense, because doomed transactions won't commit, but a couple of subtle things broke: 1. If all uncommitted r/w transactions are doomed, a READ ONLY transaction would arbitrarily not benefit from the safe snapshot optimization. It would not be taken immediately, and yet no other transaction would set SXACT_FLAG_RO_SAFE later. 2. In the same circumstances but with DEFERRABLE, GetSafeSnapshot() would correctly exit its wait loop without sleeping and then take the optimization in non-assert builds, but assert builds would fail a sanity check that SXACT_FLAG_RO_SAFE had been set by another transaction. This is similar to the case for PredXact->WritableSxactCount == 0. We should opt out immediately if our possibleUnsafeConflicts list is empty after filtering. The code to maintain the serializable global xmin is moved down below the new opt out site, because otherwise we'd have to reverse its effects before returning. Back-patch to all supported releases. Bug #17368. Reported-by: Alexander Lakhin <exclusion@gmail.com> Discussion: https://postgr.es/m/17116-d6ca217acc180e30%40postgresql.org Discussion: https://postgr.es/m/20110707212159.GF76634%40csail.mit.edu
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: https://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 https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.