函数效果如下：

select * from cxf limit 10;
id
----------
a
b
c
d
e
f
g
h
i
j
(10 row)

MysqL=# select STRCAT2(id,'|') from (select * from cxf limit 10) t;
                                         strcat2                                         
-----------------------------------------------------------------------------------------
 a|b|c|d|e|f|g|h|i|j
(1 row)

创建聚集函数的子函数：

drop FUNCTION strcat2_sfunc(varchar,varchar,varchar);
CREATE FUNCTION strcat2_sfunc(varchar,varchar)
RETURNS varchar AS '/home/MysqL/cxf/postgresql-8.2.16/contrib/strcat/libstrcat2_sfunc'
LANGUAGE C IMMUTABLE;

创建聚集函数

drop AGGREGATE PUBLIC.STRCAT2(VARCHAR,varchar);
CREATE  AGGREGATE PUBLIC.STRCAT2(VARCHAR,varchar)
(
  SFUNC=strcat2_sfunc,STYPE=VARCHAR  
);

子函数源码如下：

strcat2_sfunc.c


#include "postgres.h"
#include "funcapi.h"
#include "fmgr.h"
#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

PG_FUNCTION_INFO_V1(strcat2_sfunc);
Datum
strcat2_sfunc(PG_FUNCTION_ARGS)
{
                VarChar  *arg1;
                VarChar  *arg2;
                VarChar  *arg3;
                if(PG_ARGISNULL(0) && PG_ARGISNULL(1))
                {
                         PG_RETURN_NULL();
                }
                else if(PG_ARGISNULL(0)){
                                PG_RETURN_VARCHAR_P(PG_GETARG_VARCHAR_P(1));
                }
                else if(PG_ARGISNULL(1))
                {
                                PG_RETURN_VARCHAR_P(PG_GETARG_VARCHAR_P(0));
                }
                else{
            arg1 = PG_GETARG_VARCHAR_P(0);
            arg2 = PG_GETARG_VARCHAR_P(1);
            arg3 = PG_GETARG_VARCHAR_P(2); 
            int32 new_text_size = VARSIZE(arg1) + VARSIZE(arg2) +VARSIZE(arg3)- VARHDRSZ*2;
            VarChar *new_text = (VarChar *) palloc(new_text_size);
 
            VARATT_SIZEP(new_text) = new_text_size;
            memcpy(VARDATA(new_text),VARDATA(arg1),VARSIZE(arg1) - VARHDRSZ);
            memcpy(VARDATA(new_text) + (VARSIZE(arg1) - VARHDRSZ),VARDATA(arg3),VARSIZE(arg3) - VARHDRSZ);
            memcpy(VARDATA(new_text) + (VARSIZE(arg1)+VARSIZE(arg3) - VARHDRSZ*2),VARDATA(arg2),VARSIZE(arg2)- VARHDRSZ);
            PG_RETURN_VARCHAR_P(new_text);
  }
}

Makefile:

#

# Makefile for building Postgresql extension modules

#

MODULE_big = strcat2_sfunc
OBJS = strcat2_sfunc.o

DATA = 

DOCS = 

REGRESS = 

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