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VACUUM updates leaf-level FSM entries immediately after cleaning the corresponding heap blocks. fsmpage.c updates the intra-page search trees on the leaf-level FSM pages when this happens, but it does not touch the upper-level FSM pages, so that the released space might not actually be findable by searchers. Previously, updating the upper-level pages happened only at the conclusion of the VACUUM run, in a single FreeSpaceMapVacuum() call. This is bad because the VACUUM might get canceled before ever reaching that point, so that from the point of view of searchers no space has been freed at all, leading to table bloat. We can improve matters by updating the upper pages immediately after each cycle of index-cleaning and heap-cleaning, processing just the FSM pages corresponding to the range of heap blocks we have now fully cleaned. This adds a small amount of extra work, since the FSM pages leading down to each range boundary will be touched twice, but it's pretty negligible compared to everything else going on in a large VACUUM. If there are no indexes, VACUUM doesn't work in cycles but just cleans each heap page on first visit. In that case we just arbitrarily update upper FSM pages after each 8GB of heap. That maintains the goal of not letting all this work slide until the very end, and it doesn't seem worth expending extra complexity on a case that so seldom occurs in practice. In either case, the FSM is fully up to date before any attempt is made to truncate the relation, so that the most likely scenario for VACUUM cancellation no longer results in out-of-date upper FSM pages. When we do successfully truncate, adjusting the FSM to reflect that is now fully handled within FreeSpaceMapTruncateRel. Claudio Freire, reviewed by Masahiko Sawada and Jing Wang, some additional tweaks by me Discussion: https://postgr.es/m/CAGTBQpYR0uJCNTt3M5GOzBRHo+-GccNO1nCaQ8yEJmZKSW5q1A@mail.gmail.com |
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README
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/.