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Previously, the right to split a bucket was represented by a heavyweight lock on the page number of the primary bucket page. Unfortunately, this meant that every scan needed to take a heavyweight lock on that bucket also, which was bad for concurrency. Instead, use a cleanup lock on the primary bucket page to indicate the right to begin a split, so that scans only need to retain a pin on that page, which is they would have to acquire anyway, and which is also much cheaper. In addition to reducing the locking cost, this also avoids locking out scans and inserts for the entire lifetime of the split: while the new bucket is being populated with copies of the appropriate tuples from the old bucket, scans and inserts can happen in parallel. There are minor concurrency improvements for vacuum operations as well, though the situation there is still far from ideal. This patch also removes the unworldly assumption that a split will never be interrupted. With the new code, a split is done in a series of small steps and the system can pick up where it left off if it is interrupted prior to completion. While this patch does not itself add write-ahead logging for hash indexes, it is clearly a necessary first step, since one of the things that could interrupt a split is the removal of electrical power from the machine performing it. Amit Kapila. I wrote the original design on which this patch is based, and did a good bit of work on the comments and README through multiple rounds of review, but all of the code is Amit's. Also reviewed by Jesper Pedersen, Jeff Janes, and others. Discussion: http://postgr.es/m/CAA4eK1LfzcZYxLoXS874Ad0+S-ZM60U9bwcyiUZx9mHZ-KCWhw@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/.