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This avoids a very significant amount of buffer manager traffic and contention when scanning hash indexes, because it's no longer necessary to lock and pin the metapage for every scan. We do need some way of figuring out when the cache is too stale to use any more, so that when we lock the primary bucket page to which the cached metapage points us, we can tell whether a split has occurred since we cached the metapage data. To do that, we use the hash_prevblkno field in the primary bucket page, which would otherwise always be set to InvalidBuffer. This patch contains code so that it will continue working (although less efficiently) with hash indexes built before this change, but perhaps we should consider bumping the hash version and ripping out the compatibility code. That decision can be made later, though. Mithun Cy, reviewed by Jesper Pedersen, Amit Kapila, and by me. Before committing, I made a number of cosmetic changes to the last posted version of the patch, adjusted _hash_getcachedmetap to be more careful about order of operation, and made some necessary updates to the pageinspect documentation and regression tests.
The PostgreSQL contrib tree
---------------------------
This subtree contains porting tools, analysis utilities, and plug-in
features that are not part of the core PostgreSQL system, mainly
because they address a limited audience or are too experimental to be
part of the main source tree. This does not preclude their
usefulness.
User documentation for each module appears in the main SGML
documentation.
When building from the source distribution, these modules are not
built automatically, unless you build the "world" target. You can
also build and install them all by running "make all" and "make
install" in this directory; or to build and install just one selected
module, do the same in that module's subdirectory.
Some directories supply new user-defined functions, operators, or
types. To make use of one of these modules, after you have installed
the code you need to register the new SQL objects in the database
system by executing a CREATE EXTENSION command. In a fresh database,
you can simply do
CREATE EXTENSION module_name;
See the PostgreSQL documentation for more information about this
procedure.