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An nbtree split point can be thought of as a point between two adjoining tuples from an imaginary version of the page being split that includes the incoming/new item (in addition to the items that really are on the page). These adjoining tuples are called the lastleft and firstright tuples. The variables that represent split points contained a field called firstright, which is an offset number of the first data item from the original page that goes on the new right page. The corresponding tuple from origpage was usually the same thing as the actual firstright tuple, but not always: the firstright tuple is sometimes the new/incoming item instead. This situation seems unnecessarily confusing. Make things clearer by renaming the origpage offset returned by _bt_findsplitloc() to "firstrightoff". We now have a firstright tuple and a firstrightoff offset number which are comparable to the newitem/lastleft tuples and the newitemoff/lastleftoff offset numbers respectively. Also make sure that we are consistent about how we describe nbtree page split point state. Push the responsibility for dealing with pg_upgrade'd !heapkeyspace indexes down to lower level code, relieving _bt_split() from dealing with it directly. This means that we always have a palloc'd left page high key on the leaf level, no matter what. This enables simplifying some of the code (and code comments) within _bt_split(). Finally, restructure the page split code to make it clearer why suffix truncation (which only takes place during leaf page splits) is completely different to the first data item truncation that takes place during internal page splits. Tuples are marked as having fewer attributes stored in both cases, and the firstright tuple is truncated in both cases, so it's easy to imagine somebody missing the distinction.
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.