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Commit 0d861bbb, which introduced deduplication to nbtree, added some logic to take large posting list tuples into account when choosing a split point. We subtract firstright posting list overhead from the projected new high key size when calculating leftfree/rightfree values for an affected candidate split point. Posting list tuples aren't special to nbtsplitloc.c, but taking them into account like this makes a huge difference in practice. Posting list tuples are frequently tuple size outliers. However, commit 0d861bbb missed a closely related issue: split interval itself is calculated based on the assumption that tuples on the page being split are roughly equisized. That assumption was acceptable back when commit fab25024 taught the logic for choosing a split point about suffix truncation, but it's pretty questionable now that very large tuple sizes are common. This oversight led to unbalanced page splits in low cardinality multi-column indexes when deduplication was used: page splits that don't give sufficient weight to how unbalanced the split is when the interval happens to include some large posting list tuples (and when most other tuples on the page are not so large). Nail this down by calculating an initial split interval in a way that's attuned to the actual cost that we want to keep under control (not a fuzzy proxy for the cost): apply a leftfree + rightfree evenness test to each candidate split point that actually gets included in the split interval (for the default strategy). This replaces logic that used a percentage of all legal split points for the page as the basis of the initial split interval. Discussion: https://postgr.es/m/CAH2-WznJt5aT2uUB2Bs+JBLdwe0XTX67+xeLFcaNvCKxO=QBVQ@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: 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/.