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4b25f5d0ba
_bt_delitems_vacuum() comments claimed that it isn't worth another scan
of the page to avoid falsely unsetting the BTP_HAS_GARBAGE page flag
hint (this happens to be the same wording that was removed from
_bt_delitems_delete() by my recent commit fe97c61c). The comments made
little sense, though. The issue can't have much to do with performing a
second scan of the target leaf page, since an LP_DEAD test could easily
be performed in the first scan of the page anyway (the scan that takes
place in btvacuumpage() caller).
Revise the explanation. It makes much more sense to frame this as an
issue about recovery conflicts. _bt_delitems_vacuum() cannot easily
generate an XID cutoff in the same way that _bt_delitems_delete() is
designed to.
Falsely unsetting the page flag is not ideal, and is likely to happen
more often than was supposed by the original comments. Explain why it
usually isn't a problem in practice. There may be an argument for
_bt_delitems_vacuum() not clearing the BTP_HAS_GARBAGE bit, removing the
question of it being falsely unset by VACUUM (there may even be an
argument for not using a page level hint at all). This can be revisited
later.
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/.