我目前了解的PG插件大约有两种，一种是利用hook，另一种是建立C函数，然后在数据库中进行关联。PG本身就是最好的老师，大家有兴趣可以看一下contrib目录下的插件。
下面将对两种方式进行介绍：
1、利用hook建立插件，hook是PG中可以对PG运行机制进行修改的一种方式，大家可以看一下我之前对：
a.在contrib目录下建立brother目录，建立brother.c文件和Makefile文件。具体内容见下：
brother.c（在用户brother登录验证成功后，显示信息Welcome to the world of PostgreSQL!）:
/*------------------------------------------------------------------------- * * brother.c * welcome to the world of PostgreSQL * * contrib/brother/brother.c *------------------------------------------------------------------------- */#include "postgres.h"#include "libpq/auth.h"#include "miscadmin.h"#include "utils/guc.h"PG_MODULE_MAGIC;void _PG_init(void);void _PG_fini(void);static ClientAuthentication_hook_type pre_ClientAuthentication_hook = NULL;staticvoid welcome_to_pg(Port *port, int status){ if(status == STATUS_OK && (strcmp(port->user_name, "brother") == 0)) printf("Welcome to the world of PostgreSQL!\n");}void_PG_init(void){ pre_ClientAuthentication_hook = ClientAuthentication_hook; ClientAuthentication_hook = welcome_to_pg;}void_PG_fini(void){ ClientAuthentication_hook = pre_ClientAuthentication_hook;} Makefile文件： # contrib/brother/MakefileMODULE_big = brotherOBJS = brother.o# uncomment the following two lines to enable cracklib support# PG_CPPFLAGS = -DUSE_CRACKLIB '-DCRACKLIB_DICTPATH="/usr/lib/cracklib_dict"'# SHLIB_LINK = -lcrackifdef USE_PGXSPG_CONFIG = pg_configPGXS := $(shell $(PG_CONFIG) --pgxs)include $(PGXS)elsesubdir = contrib/brothertop_builddir = ../..include $(top_builddir)/src/Makefile.globalinclude $(top_srcdir)/contrib/contrib-global.mkendifb.编译并安装
[postgres@localhost brother]$ makegcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -g -O2 -fpic -I. -I. -I../../src/include -D_GNU_SOURCE -c -o brother.o brother.cgcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -g -O2 -fpic -shared -o brother.so brother.o -L../../src/port -L../../src/common -Wl,--as-needed -Wl,-rpath,'/opt/hg3.0/lib',--enable-new-dtags [postgres@localhost brother]$ make install/bin/mkdir -p '/opt/hg3.0/lib/postgresql'/usr/bin/install -c -m 755 brother.so '/opt/hg3.0/lib/postgresql/brother.so'c.文件安装位置
[postgres@localhost postgresql]$ pwd/opt/hg3.0/lib/postgresql[postgres@localhost postgresql]$ ll brother.so -rwxr-xr-x. 1 postgres postgres 21523 Dec 23 06:27 brother.sod.在postgresql.conf中配置
shared_preload_libraries = 'brother'e.运行，查看效果
[postgres@localhost bin]$ ./pg_ctl -D ../data startserver starting[postgres@localhost bin]$ LOG: database system was shut down at 2015-12-23 06:32:38 PSTLOG: MultiXact member wraparound protections are now enabledLOG: database system is ready to accept connectionsLOG: autovacuum launcher started[postgres@localhost bin]$ ./psqlpsql (9.4.5)Type "help" for help.postgres=# create user brother;CREATE ROLE[postgres@localhost bin]$ ./psql postgres brotherWelcome to the world of PostgreSQL!psql (9.4.5)Type "help" for help.postgres=>
2、建立C函数，并建立关联
a.在contrib下建立目录userid，建立C文件userid.c，建立Makefile文件，建立两个SQL文件，userid--1.0.sql，userid--unpackaged--1.0.sql，以及一个control文件，userid.control。
(1)C文件和Makefile就不介绍了，内容如下：
userid.c：/*------------------------------------------------------------------------- * * userid.c * display current session user oid * * contrib/userid/userid.c *------------------------------------------------------------------------- */#include "postgres.h"#include "libpq/auth.h"#include "utils/guc.h"#include "miscadmin.h"PG_MODULE_MAGIC;PG_FUNCTION_INFO_V1(get_current_user_id);Datumget_current_user_id(PG_FUNCTION_ARGS){ PG_RETURN_OID(GetSessionUserId());} Makefile： # contrib/userid/MakefileMODULE_big = useridOBJS = userid.oEXTENSION = useridDATA = userid--1.0.sql userid--unpackaged--1.0.sqlifdef USE_PGXSPG_CONFIG = pg_configPGXS := $(shell $(PG_CONFIG) --pgxs)include $(PGXS)elsesubdir = contrib/useridtop_builddir = ../..include $(top_builddir)/src/Makefile.globalinclude $(top_srcdir)/contrib/contrib-global.mkendif
(2)userid--1.0.sql是在create extension时加载的SQL文件，会执行SQL文件中的sql，内容如下：
/* contrib/userid/userid--1.0.sql */-- complain if script is sourced in psql, rather than via CREATE EXTENSION\echo Use "CREATE EXTENSION userid" to load this file. \quit-- Register the function.CREATE SCHEMA brother;CREATE FUNCTION get_current_userid()RETURNS OIDAS 'MODULE_PATHNAME', 'get_current_user_id'LANGUAGE C;(3) userid--unpackaged--1.0.sql是CREATE EXTENSION userid FROM unpackaged时调用的SQL文件，这个文件是在更新9.1之前版本的插件函数时使用的，后面会介绍用法。
/* contrib/userid/userid--unpackaged--1.0.sql */-- complain if script is sourced in psql, rather than via CREATE EXTENSION\echo Use "CREATE EXTENSION userid FROM unpackaged" to load this file. \quitALTER EXTENSION userid ADD schema brother;ALTER EXTENSION userid ADD function get_current_userid();(4)control文件是在 create extension加载的文件，根据这个文件，PG去寻找对应的sql文件，内容如下：
# userid extensioncomment = 'display current session user oid'default_version = '1.0'module_pathname = '$libdir/userid'relocatable = trueb.编译安装
[postgres@localhost userid]$ makegcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -g -O2 -fpic -I. -I. -I../../src/include -D_GNU_SOURCE -c -o userid.o userid.cgcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -g -O2 -fpic -shared -o userid.so userid.o -L../../src/port -L../../src/common -Wl,--as-needed -Wl,-rpath,'/opt/hg3.0/lib',--enable-new-dtags [postgres@localhost userid]$ make install/bin/mkdir -p '/opt/hg3.0/lib/postgresql'/bin/mkdir -p '/opt/hg3.0/share/postgresql/extension'/bin/mkdir -p '/opt/hg3.0/share/postgresql/extension'/usr/bin/install -c -m 755 userid.so '/opt/hg3.0/lib/postgresql/userid.so'/usr/bin/install -c -m 644 userid.control '/opt/hg3.0/share/postgresql/extension/'/usr/bin/install -c -m 644 userid--1.0.sql userid--unpackaged--1.0.sql '/opt/hg3.0/share/postgresql/extension/'c.文件安装位置
[postgres@localhost postgresql]$ pwd/opt/hg3.0/lib/postgresql[postgres@localhost postgresql]$ ll userid.so -rwxr-xr-x. 1 postgres postgres 18665 Dec 23 06:48 userid.so[postgres@localhost extension]$ pwd/opt/hg3.0/share/postgresql/extension[postgres@localhost extension]$ ll userid*-rw-r--r--. 1 postgres postgres 329 Dec 23 06:48 userid--1.0.sql-rw-r--r--. 1 postgres postgres 142 Dec 23 06:48 userid.control-rw-r--r--. 1 postgres postgres 304 Dec 23 06:48 userid--unpackaged--1.0.sqld.配置
[postgres@localhost bin]$ ./psql psql (9.4.5)Type "help" for help.postgres=# create extension userid;CREATE EXTENSIONpostgres=# select * from pg_extension ; extname | extowner | extnamespace | extrelocatable | extversion | extconfig | extcondition --------------------+----------+--------------+----------------+------------+-----------+-------------- plpgsql | 10 | 11 | f | 1.0 | | pgfincore | 10 | 2200 | t | 1.1.1 | | pg_stat_statements | 10 | 2200 | t | 1.2 | | pg_freespacemap | 10 | 2200 | t | 1.0 | | pgstattuple | 10 | 2200 | t | 1.2 | | userid | 10 | 2200 | t | 1.0 | | (6 rows)e.运行，查看效果
postgres=# select SESSION_USER; session_user -------------- postgres(1 row)postgres=# select oid from pg_authid where rolname = 'postgres'; oid ----- 10(1 row)postgres=# select get_current_userid(); get_current_userid -------------------- 10(1 row)postgres=# \dn List of schemas Name | Owner ---------+---------- brother | postgres public | postgres(2 rows)postgres=#f.使用unpackaged.sql，这里是更新9.1之前的插件使用的，现在来模拟一下：
postgres=# create schema brother;CREATE SCHEMApostgres=# CREATE FUNCTION get_current_userid()RETURNS OIDAS '$libdir/userid', 'get_current_user_id'LANGUAGE C;CREATE FUNCTIONpostgres=# create extension userid with schema public from unpackaged;CREATE EXTENSION因为在9.1之前都是利用SQL安装的插件，没有和extension进行关联，so，在这里可以关联一下。那什么是关联呢？
g.extension关联的意思，首先大家需要知道PG的表，pg_depend。这张表是当我们进行drop时，添加参数CASCADE会调用查询的一张表，这张表主要存储对象oid依赖的对象oid等等。下面来看一下：
postgres=# select oid from pg_proc where proname = 'get_current_userid'; oid ------- 24715(1 row)postgres=# select oid from pg_namespace where nspname = 'brother'; oid ------- 24714(1 row)postgres=# select oid from pg_extension where extname = 'userid'; oid ------- 24716(1 row)postgres=# select * from pg_depend where refobjid = 24716; classid | objid | objsubid | refclassid | refobjid | refobjsubid | deptype ---------+-------+----------+------------+----------+-------------+--------- 2615 | 24714 | 0 | 3079 | 24716 | 0 | e 1255 | 24715 | 0 | 3079 | 24716 | 0 | e(2 rows)这个地方就是当你drop extension时，同时会删除你插件中SQL文件建立的对象的原因。那当create extension时，又是什么地方去写pg_depend信息的呢？
h.当你进行create extension时，下面这段代码就是自动建立关联的代码：
/* * If we are executing a CREATE EXTENSION operation, mark the given object * as being a member of the extension. Otherwise, do nothing. * * This must be called during creation of any user-definable object type * that could be a member of an extension. * * If isReplace is true, the object already existed (or might have already * existed), so we must check for a pre-existing extension membership entry. * Passing false is a guarantee that the object is newly created, and so * could not already be a member of any extension. */voidrecordDependencyOnCurrentExtension(const ObjectAddress *object, bool isReplace){ /* Only whole objects can be extension members */ Assert(object->objectSubId == 0); if (creating_extension) { ObjectAddress extension; /* Only need to check for existing membership if isReplace */ if (isReplace) { Oid oldext; oldext = getExtensionOfObject(object->classId, object->objectId); if (OidIsValid(oldext)) { /* If already a member of this extension, nothing to do */ if (oldext == CurrentExtensionObject) return; /* Already a member of some other extension, so reject */ ereport(ERROR, (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE), errmsg("%s is already a member of extension \"%s\"", getObjectDescription(object), get_extension_name(oldext)))); } } /* OK, record it as a member of CurrentExtensionObject */ extension.classId = ExtensionRelationId; extension.objectId = CurrentExtensionObject; extension.objectSubId = 0; recordDependencyOn(object, &extension, DEPENDENCY_EXTENSION); }} 至此，就是我在学习PG源码中学习到的知识。这里介绍的还比较简单，请大家多多关注，我后续还会继续整理的。