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‎201605/20160529_03.md‎

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## PostgreSQL ECPG 开发 DEMO
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### 作者
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digoal
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### 日期
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2016-05-29
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### 标签
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PostgreSQL , ECPG
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----
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## 背景
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ECPG 是在C中嵌套SQL的一种用法。
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写好pgc文件后，需要使用ecpg程序将pgc编程成C文件来使用。
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![1](20160529_03_pic_001.png)
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详细的用法请参考
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https://www.postgresql.org/docs/9.5/static/ecpg.html
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ecpg的用法, 以EXEC SQL开头表示后面是SQL写法
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一些简单的用法
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大小写敏感.
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1\. 连接数据库
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```
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EXEC SQL CONNECT TO target [AS connection-name] [USER user-name];
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target :
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dbname[@hostname][:port]
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tcp:postgresql://hostname[:port][/dbname][?options]
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unix:postgresql://hostname[:port][/dbname][?options]
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例子
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EXEC SQL CONNECT TO [email protected];
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EXEC SQL CONNECT TO unix:postgresql://sql.mydomain.com/mydb AS myconnection USER john;
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EXEC SQL BEGIN DECLARE SECTION;
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const char *target = "[email protected]";
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const char *user = "john";
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const char *passwd = "secret";
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EXEC SQL END DECLARE SECTION;
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...
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EXEC SQL CONNECT TO :target USER :user USING :passwd;
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/* or EXEC SQL CONNECT TO :target USER :user/:passwd; */
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```
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2\. 定义ecpg变量
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PostgreSQL数据类型与ecpg使用的类型的映射关系
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https://www.postgresql.org/docs/9.5/static/ecpg-variables.html#ECPG-VARIABLES-TYPE-MAPPING
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有些类型没有一一对应的关系，需要使用ecpg的函数来转换。
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下面是一个DEMO
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首先需要编写pgc文件。
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```
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$ vi t.pgc
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#include <stdio.h>
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#include <stdlib.h>
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#include <pgtypes_numeric.h>
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EXEC SQL WHENEVER SQLERROR STOP;
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int
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main(void)
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{
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EXEC SQL BEGIN DECLARE SECTION;
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numeric *num;
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numeric *num2;
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decimal *dec;
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EXEC SQL END DECLARE SECTION;
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EXEC SQL CONNECT TO tcp:postgresql://xxxcs.com:3433/postgres AS db_digoal USER digoal USING digoal;
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num = PGTYPESnumeric_new();
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dec = PGTYPESdecimal_new();
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EXEC SQL SELECT 12.345::numeric(4,2), 23.456::decimal(4,2) INTO :num, :dec;
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printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 0));
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printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 1));
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printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 2));
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/* Convert decimal to numeric to show a decimal value. */
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num2 = PGTYPESnumeric_new();
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PGTYPESnumeric_from_decimal(dec, num2);
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printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 0));
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printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 1));
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printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 2));
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PGTYPESnumeric_free(num2);
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PGTYPESdecimal_free(dec);
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PGTYPESnumeric_free(num);
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EXEC SQL COMMIT;
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EXEC SQL DISCONNECT ALL;
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return 0;
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}
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```
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本地环境中需要有依赖的头文件和库.
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编译.pgc
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-t 表示使用自动提交
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```
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ecpg -t -c -I/home/digoal/pgsql9.6/include -o t.c t.pgc
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```
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查看编译后的.c
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```
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/* Processed by ecpg (4.12.0) */
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/* These include files are added by the preprocessor */
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#include <ecpglib.h>
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#include <ecpgerrno.h>
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#include <sqlca.h>
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/* End of automatic include section */
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#line 1 "t.pgc"
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#include <stdio.h>
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#include <stdlib.h>
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#include <pgtypes_numeric.h>
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/* exec sql whenever sqlerror stop ; */
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#line 5 "t.pgc"
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int
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main(void)
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{
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/* exec sql begin declare section */
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#line 11 "t.pgc"
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numeric * num ;
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#line 12 "t.pgc"
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numeric * num2 ;
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#line 13 "t.pgc"
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decimal * dec ;
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/* exec sql end declare section */
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#line 14 "t.pgc"
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{ ECPGconnect(__LINE__, 0, "tcp:postgresql://rdsqm2ffv0wjxnxk5nbsi.pg.rds.aliyuncs.com:3433/postgres" , "digoal" , "digoal" , "db_digoal", 1);
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#line 16 "t.pgc"
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if (sqlca.sqlcode < 0) exit (1);}
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#line 16 "t.pgc"
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num = PGTYPESnumeric_new();
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dec = PGTYPESdecimal_new();
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{ ECPGdo(__LINE__, 0, 1, NULL, 0, ECPGst_normal, "select 12.345 :: numeric ( 4 , 2 ) , 23.456 :: decimal ( 4 , 2 )", ECPGt_EOIT,
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ECPGt_numeric,&(num),(long)1,(long)0,sizeof(numeric),
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ECPGt_NO_INDICATOR, NULL , 0L, 0L, 0L,
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ECPGt_decimal,&(dec),(long)1,(long)0,sizeof(decimal),
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ECPGt_NO_INDICATOR, NULL , 0L, 0L, 0L, ECPGt_EORT);
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#line 21 "t.pgc"
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if (sqlca.sqlcode < 0) exit (1);}
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#line 21 "t.pgc"
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printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 0));
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printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 1));
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printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 2));
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/* Convert decimal to numeric to show a decimal value. */
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num2 = PGTYPESnumeric_new();
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PGTYPESnumeric_from_decimal(dec, num2);
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printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 0));
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printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 1));
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printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 2));
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PGTYPESnumeric_free(num2);
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PGTYPESdecimal_free(dec);
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PGTYPESnumeric_free(num);
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{ ECPGtrans(__LINE__, NULL, "commit");
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#line 39 "t.pgc"
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if (sqlca.sqlcode < 0) exit (1);}
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#line 39 "t.pgc"
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{ ECPGdisconnect(__LINE__, "ALL");
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#line 40 "t.pgc"
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if (sqlca.sqlcode < 0) exit (1);}
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#line 40 "t.pgc"
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return 0;
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}
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```
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编译，链接 .c
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```
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gcc -I/home/digoal/pgsql9.6/include -Wall -g t.c -L/home/digoal/pgsql9.6/lib -lecpg -lpq -lpgtypes -o t
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```
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也可以写成Makefile如下 :
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```
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$ vi Makefile
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```
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注意行首缩进的地方需要TAB键输入.
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```
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ECPG = ecpg
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CC = gcc
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INCLUDES = -I$(shell pg_config --includedir)
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LIBPATH = -L$(shell pg_config --libdir)
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CFLAGS += $(INCLUDES)
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LDFLAGS += -Wall -g
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LDLIBS += $(LIBPATH) -lecpg -lpq -lpgtypes
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%.c: %.pgc
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$(ECPG) -t -c $(INCLUDES) -o $@ $<
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%: %.o
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$(CC) $(CFLAGS) $(LDFLAGS) $(LDLIBS) -o $@ $<
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TESTS = t t.c
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default: $(TESTS)
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clean:
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rm -f *.o *.so t t.c
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```
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使用时注意把pg_config弄进PATH
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```
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$ export PATH=/home/digoal/pgsql9.6/bin:$PATH
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$ make
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```
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使用编译好的t :
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```
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./t
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numeric = 12
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numeric = 12.4
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numeric = 12.35
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decimal = 23
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decimal = 23.5
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decimal = 23.46
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```
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‎201605/20160529_03_pic_001.png‎

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‎201605/readme.md‎

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### 文章列表
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----
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##### 20160531_01.md [《PostgreSQL 函数稳定性与constraint_excluded分区表逻辑推理过滤的CASE》](20160531_01.md)
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##### 20160529_03.md [《PostgreSQL ECPG 开发 DEMO》](20160529_03.md)
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##### 20160529_02.md [《EDB PPAS的"坑" 不兼容PostgreSQL一例》](20160529_02.md)
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##### 20160529_01.md [《PostgreSQL 批量权限 管理方法》](20160529_01.md)
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##### 20160528_01.md [《深入浅出PostgreSQL B-Tree索引结构》](20160528_01.md)

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