A bit of background. The core contrib extensions generally rely on a relative path to include the core Makefiles needed to build the extension. Because they ship with the core distribution, they can generally expect that the core has already been compiled, the necessary Makefiles have been created, and that they should be built against them. All the assumptions are that the extensions should be built against the source tree in which they are distributed. So there is no need to use pg_config to find PGXS; it already knows where to find what it needs.

But as extensions, there is still the possibility that one might want to build them against an existing installation of PostgreSQL, or an older version than the source with which they’re distributed. So the core hackers provided the USE_PGXS variable so that one can in effect tell make, “Don’t build against the local source tree, but find PGXS for some other install and build against that, instead.” It was expected to be exceptional, since most folks would build against the local source tree, and not a big deal to make anyone else build it with:

make USE_PGXS=1

Today things are different. There is a growing ecosystem of third party extensions on PGXN, pgFoundry, GitHub, and Bitbucket, and obviously they’re not distributed with the PostgreSQL core. For these extensions, there is no surrounding PostgreSQL source code to automatically include, so they must use pg_config to find PGXS in order build.

Yet, there are quite a few third-party extensions that nevertheless assume that they are in the contrib directory of the PostgreSQL source code distribution, and so still have the USE_PGXS variable. The twitter_ftw 1.0.0 Makefile is a recent example. Just like core extensions, it has this code:

ifdef USE_PGXS
PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)
else
subdir = contrib/twitter_fdw
top_builddir = ../..
include $(top_builddir)/src/Makefile.global
include $(top_srcdir)/contrib/contrib-global.mk
endif

Because Hitoshi-san originally copied the Makefile from a core extension, it still assumes it will be distributed in core by default. And as I said, there are quite a few third-party extensions that exhibit this pattern.

And now the PSA: Please don’t use USE_PGXS in PostgreSQL extension Makefiles.

Not only is it unnecessary, it makes no sense for third-party extensions. They should always assume that they need to use pg_config to find PGXS. If you have an extension Makefile with USE_PGXS like twitter_ftw 1.0.0 did, you should change it to something like this (as Hitoshi-san did in the twitter_ftw 1.0.1 Makefile):

PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)

That’s it. I am asking you to make your Makefile simpler.

An Aside

There is one situation in which you might need to include the core contrib Makefiles. And it’s pretty unusual. If you need to support PostgreSQL 8.1 or earlier, pg_config will not be able to tell you where to find PGXS. So users will have to copy the extension source directory into the PostgreSQL source contrib/ directory and build from there. They will need a way to tell make not to use PGXS. In this one unusual case, I suggest you add a NO_PGXS variable. pgTAP’s Makefile provides an example. But honestly, very few extensions need to support PostgreSQL 8.1 (the oldest release currently supported by the core hackers is 8.3!), so make use of this pattern only if absolutely necessary.

Otherwise, please don’t use USE_PGXS.

If you want a complete guide to creating your extension Makefile, have a look at the PGXN Howto, which includes some detailed examples that include support for pre- and post-CREATE EXTENSION support. The PGXS docs contain additional details about all the Makefile variables you can use to simplify extension configuration and installation. Check ’em out.