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Hstore data generated on pg 8.4 and pg_upgraded to current versions remains in its original on-disk format unless modified. The same goes for values generated by the addon hstore-new module on pre-9.0 versions. (The hstoreUpgrade function converts old values on the fly when read in, but the on-disk value is not modified by this.) Since old-format empty hstores (and hstore-new hstores) have representations compatible with the new format, hstoreUpgrade thought it could get away without modifying such values; but this breaks hstore_hash (and the new hstore_hash_extended) which assumes bit-perfect matching between semantically identical hstore values. Only one bit actually differs (the "new version" flag in the count field) but that of course is enough to break the hash. Fix by making hstoreUpgrade unconditionally convert all old values to new format. Backpatch all the way, even though this changes a hash value in some cases, because in those cases the hash value is already failing - for example, a hash join between old- and new-format empty hstores will be failing to match, or a hash index on an hstore column containing an old-format empty value will be failing to find the value since it will be searching for a hash derived from a new-format datum. (There are no known field reports of this happening, probably because hashing of hstores has only been useful in limited circumstances and there probably isn't much upgraded data being used this way.) Per concerns arising from discussion of commit eb6f29141be. Original bug is my fault. Discussion: https://postgr.es/m/60b1fd3b-7332-40f0-7e7f-f2f04f777747%402ndquadrant.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.