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PageIndexTupleOverwrite performs approximately the same function as PageIndexTupleDelete (or PageIndexDeleteNoCompact) followed by PageAddItem targeting the same item pointer offset. But in the case where the new tuple is the same size as the old, it avoids shuffling other data around on the page, because the new tuple is placed where the old one was rather than being appended to the end of the page. This has been shown to provide a substantial speedup for some GiST use-cases. Also, this change allows some API simplifications: we can get rid of the rather klugy and error-prone PAI_ALLOW_FAR_OFFSET flag for PageAddItemExtended, since that was used only to cover a corner case for BRIN that's better expressed by using PageIndexTupleOverwrite. Note that this patch causes a rather subtle WAL incompatibility: the physical page content change represented by certain WAL records is now different than it was before, because while the tuples have the same itempointer line numbers, the tuples themselves are in different places. I have not bumped the WAL version number because I think it doesn't matter unless you are trying to do bitwise comparisons of original and replayed pages, and in any case we're early in a devel cycle and there will probably be more WAL changes before v10 gets out the door. There is probably room to make use of PageIndexTupleOverwrite in SP-GiST and GIN too, but that is left for a future patch. Andrey Borodin, reviewed by Anastasia Lubennikova, whacked around a bit by me Discussion: <CAJEAwVGQjGGOj6mMSgMwGvtFd5Kwe6VFAxY=uEPZWMDjzbn4VQ@mail.gmail.com> |
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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/.