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someone else has just updated it, we have to set priorXmax to that tuple's xmax (ie, the XID of the other xact that updated it) before looping back to examine the next tuple. Obviously, the next tuple in the update chain should have that XID as its xmin, not the same xmin as the preceding tuple that we had been trying to lock. The mismatch would cause the EvalPlanQual logic to decide that the tuple chain ended in a deletion, when actually there was a live tuple that should have been found. I inserted this error when recently adding logic to EvalPlanQual to make it lock tuples before returning them (as opposed to the old method in which the lock would occur much later, causing a great deal of work to be wasted if we only then discover someone else updated it). Sigh. Per today's report from Takahiro Itagaki of inconsistent results during pgbench runs. |
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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. Changes between all PostgreSQL releases are recorded in the file HISTORY. 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/.