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5028f22f6e
The old algorithm was found to not be the usual CRC-32 algorithm, used by Ethernet et al. We were using a non-reflected lookup table with code meant for a reflected lookup table. That's a strange combination that AFAICS does not correspond to any bit-wise CRC calculation, which makes it difficult to reason about its properties. Although it has worked well in practice, seems safer to use a well-known algorithm. Since we're changing the algorithm anyway, we might as well choose a different polynomial. The Castagnoli polynomial has better error-correcting properties than the traditional CRC-32 polynomial, even if we had implemented it correctly. Another reason for picking that is that some new CPUs have hardware support for calculating CRC-32C, but not CRC-32, let alone our strange variant of it. This patch doesn't add any support for such hardware, but a future patch could now do that. The old algorithm is kept around for tsquery and pg_trgm, which use the values in indexes that need to remain compatible so that pg_upgrade works. While we're at it, share the old lookup table for CRC-32 calculation between hstore, ltree and core. They all use the same table, so might as well.
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.