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de239d01e7
Previously this code used PageGetHeapFreeSpace on heap pages, and usually used PageGetFreeSpace on index pages (though for some reason GetHashPageStats used PageGetExactFreeSpace instead). The difference is that those functions subtract off the size of a line pointer, and PageGetHeapFreeSpace has some additional rules about returning zero if adding another line pointer would require exceeding MaxHeapTuplesPerPage. Those things make sense when testing to see if a new tuple can be put on the page, but they seem pretty strange for pure statistics collection. Additionally, statapprox_heap had a special rule about counting a "new" page as being fully available space. This also seems strange, because it's not actually usable until VACUUM or some such process initializes the page. Moreover, it's inconsistent with what pgstat_heap does, which is to count such a page as having zero free space. So make it work like pgstat_heap, which as of this patch unconditionally calls PageGetExactFreeSpace. This is more of a definitional change than a bug fix, so no back-patch. The module's documentation doesn't define exactly what "free space" means either, so we left that as-is. Frédéric Yhuel, reviewed by Rafia Sabih and Andreas Karlsson. Discussion: https://postgr.es/m/3a18f843-76f6-4a84-8cca-49537fefa15d@dalibo.com
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.