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A test case provided by Mathieu Fenniak shows that hash_seq_search'ing this hashtable can consume a very significant amount of overhead during logical decoding, which triggers frequent cache invalidation. Testing suggests that the actual population of the hashtable is often no more than a few dozen entries, so we can cut the overhead just by dropping the initial number of buckets down from 1024 --- I chose to cut it to 64. (In situations where we do have a significant number of entries, we shouldn't get any real penalty from doing this, as the dynahash.c code will resize the hashtable automatically.) This gives a further factor-of-two savings in Mathieu's test case. That may be overly optimistic for real-world benefit, as real cases may have larger average table populations, but it's hard to see it turning into a net negative for any workload. Back-patch to 9.4 where relfilenodemap.c was introduced. Discussion: https://postgr.es/m/CAHoiPjzea6N0zuCi=+f9v_j94nfsy6y8SU7-=bp4=7qw6_i=Rg@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/.