USE_PGXS 在 extension 编译中的作用

postgresql 在编译 extension时经常要使用 pgxs，如下：

# source /var/lib/pgsql/.bash_profile
# USE_PGXS=1 make  
# USE_PGXS=1 make install 

下面简单分析一下：

某个具体 contrib 的 Makefile文件

# PGXS build needs PostgreSQL 9.2 or later. Earlier versions didn't install
# plpgsql.h, so you needed the full source tree to access it.
ifdef USE_PGXS
PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)
else
subdir = contrib/xxoo
top_builddir = ../..
include $(top_builddir)/src/Makefile.global
include $(top_srcdir)/contrib/contrib-global.mk
endif

下面这句话很关键：
PGXS build needs PostgreSQL 9.2 or later

查看pg_config

$ which pg_config
/usr/pgsql-9.6/bin/pg_config

$ pg_config
BINDIR = /usr/pgsql-9.6/bin
DOCDIR = /usr/pgsql-9.6/doc
HTMLDIR = /usr/pgsql-9.6/doc/html
INCLUDEDIR = /usr/pgsql-9.6/include
PKGINCLUDEDIR = /usr/pgsql-9.6/include
INCLUDEDIR-SERVER = /usr/pgsql-9.6/include/server
LIBDIR = /usr/pgsql-9.6/lib
PKGLIBDIR = /usr/pgsql-9.6/lib
LOCALEDIR = /usr/pgsql-9.6/share/locale
MANDIR = /usr/pgsql-9.6/share/man
SHAREDIR = /usr/pgsql-9.6/share
SYSCONFDIR = /etc/sysconfig/pgsql
PGXS = /usr/pgsql-9.6/lib/pgxs/src/makefiles/pgxs.mk
CONFIGURE = '--enable-rpath' '--prefix=/usr/pgsql-9.6' '--includedir=/usr/pgsql-9.6/include' '--mandir=/usr/pgsql-9.6/share/man' '--datadir=/usr/pgsql-9.6/share' '--libdir=/usr/pgsql-9.6/lib' '--with-perl' '--with-python' '--with-tcl' '--with-tclconfig=/usr/lib64' '--with-openssl' '--with-pam' '--with-gssapi' '--with-includes=/usr/include' '--with-libraries=/usr/lib64' '--enable-nls' '--with-uuid=e2fs' '--with-libxml' '--with-libxslt' '--with-ldap' '--with-system-tzdata=/usr/share/zoneinfo' '--sysconfdir=/etc/sysconfig/pgsql' '--docdir=/usr/pgsql-9.6/doc' '--htmldir=/usr/pgsql-9.6/doc/html' 'CFLAGS=-O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=4 -m64 -mtune=generic' 'LDFLAGS=-Wl,--as-needed'
CC = gcc
CPPFLAGS = -DFRONTEND -D_GNU_SOURCE -I/usr/include/libxml2 -I/usr/include
CFLAGS = -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=4 -m64 -mtune=generic
CFLAGS_SL = -fPIC
LDFLAGS = -L../../src/common -Wl,--as-needed -L/usr/lib64 -Wl,--as-needed -Wl,-rpath,'/usr/pgsql-9.6/lib',--enable-new-dtags
LDFLAGS_EX = 
LDFLAGS_SL = 
LIBS = -lpgcommon -lpgport -lxslt -lxml2 -lpam -lssl -lcrypto -lgssapi_krb5 -lz -lreadline -lrt -lcrypt -ldl -lm  
VERSION = PostgreSQL 9.6.8

$ pg_config --pgxs
/usr/pgsql-9.6/lib/pgxs/src/makefiles/pgxs.mk

查看pgxs.mk

$ ls -l /usr/pgsql-9.6/lib/pgxs/src/makefiles/pgxs.mk
-rw-r--r-- 1 root root 8068 Feb 27 08:54 /usr/pgsql-9.6/lib/pgxs/src/makefiles/pgxs.mk

$ cat /usr/pgsql-9.6/lib/pgxs/src/makefiles/pgxs.mk
# PGXS: PostgreSQL extensions makefile

# src/makefiles/pgxs.mk

# This file contains generic rules to build many kinds of simple
# extension modules.  You only need to set a few variables and include
# this file, the rest will be done here.
#
# Use the following layout for your Makefile:
#
#   [variable assignments, see below]
#
#   PG_CONFIG = pg_config
#   PGXS := $(shell $(PG_CONFIG) --pgxs)
#   include $(PGXS)
#
#   [custom rules, rarely necessary]
#
# Set one of these three variables to specify what is built:
#
#   MODULES -- list of shared-library objects to be built from source files
#     with same stem (do not include library suffixes in this list)
#   MODULE_big -- a shared library to build from multiple source files
#     (list object files in OBJS)
#   PROGRAM -- an executable program to build (list object files in OBJS)
#
# The following variables can also be set:
#
#   EXTENSION -- name of extension (there must be a $EXTENSION.control file)
#   MODULEDIR -- subdirectory of $PREFIX/share into which DATA and DOCS files
#     should be installed (if not set, default is "extension" if EXTENSION
#     is set, or "contrib" if not)
#   DATA -- random files to install into $PREFIX/share/$MODULEDIR
#   DATA_built -- random files to install into $PREFIX/share/$MODULEDIR,
#     which need to be built first
#   DATA_TSEARCH -- random files to install into $PREFIX/share/tsearch_data
#   DOCS -- random files to install under $PREFIX/doc/$MODULEDIR
#   SCRIPTS -- script files (not binaries) to install into $PREFIX/bin
#   SCRIPTS_built -- script files (not binaries) to install into $PREFIX/bin,
#     which need to be built first
#   REGRESS -- list of regression test cases (without suffix)
#   REGRESS_OPTS -- additional switches to pass to pg_regress
#   EXTRA_CLEAN -- extra files to remove in 'make clean'
#   PG_CPPFLAGS -- will be added to CPPFLAGS
#   PG_LIBS -- will be added to PROGRAM link line
#   SHLIB_LINK -- will be added to MODULE_big link line
#   PG_CONFIG -- path to pg_config program for the PostgreSQL installation
#     to build against (typically just "pg_config" to use the first one in
#     your PATH)
#
# Better look at some of the existing uses for examples...

pgxs 是 postgrsql 9.2 之后针对编译 extension 的解决方案。如果是之前的版本，就需要 postgresql full source 完全编译。
所以，有还在使用 postgrsql 9.2之前版本的小伙伴们赶紧升级了，毕竟 postgresl 11 beta 已经开始公测了。

参考：
http://postgres.cn/docs/9.6/extend-pgxs.html