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Specific ever-decreasing insertion patterns could cause successive unbalanced nbtree page splits. Problem cases involve a large group of duplicates to the left, and ever-decreasing insertions to the right. To fix, detect the situation by considering the newitem offset before performing a split using nbtsplitloc.c's "many duplicates" strategy. If the new item was inserted just to the right of our provisional "many duplicates" split point, infer ever-decreasing insertions and fall back on a 50:50 (space delta optimal) split. This seems to barely affect cases that already had acceptable space utilization. An alternative fix also seems possible. Instead of changing nbtsplitloc.c split choice logic, we could instead teach _bt_truncate() to generate a new value for new high keys by interpolating from the lastleft and firstright key values. That would certainly be a more elegant fix, but it isn't suitable for backpatching. Discussion: https://postgr.es/m/CAH2-WznCNvhZpxa__GqAa1fgQ9uYdVc=_apArkW2nc-K3O7_NA@mail.gmail.com Backpatch: 12-, where the nbtree page split enhancements were introduced. |
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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: 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/.