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In external sort's merge phase, we maintain a binary heap holding the next tuple from each input tape. On each step, the topmost tuple is returned, and replaced with the next tuple from the same tape. We were doing the replacement by deleting the top node in one operation, and inserting the next tuple after that. However, you can do a "replace-top" operation more efficiently, in one "sift-up". A deletion will always walk the heap from top to bottom, but in a replacement, we can stop as soon as we find the right place for the new tuple. This is particularly helpful, if the tapes are not in completely random order, so that the next tuple from a tape is likely to land near the top of the heap. Peter Geoghegan, reviewed by Claudio Freire, with some editing by me. Discussion: <CAM3SWZRhBhiknTF_=NjDSnNZ11hx=U_SEYwbc5vd=x7M4mMiCw@mail.gmail.com>
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/.