安装：

进入/root/postgresql-11.2/contrib/dblink

make && make install

切换到postgres用户

[root@fce40690-0e46-4603-e80e-ca351bda31ec dblink]# su - postgres

上一次登录：三 5月  8 03:47:54 UTC 2019pts/1 上

[postgres@fce40690-0e46-4603-e80e-ca351bda31ec ~]$ psql

psql (11.2)

Type "help" for help.

postgres=# create extension dblink;

ERROR:  extension "dblink" already exists

使用：

postgres=# select dblink_connect('mydb', 'dbname=mydb host=localhost');

dblink_connect

----------------

OK

(1 row)

postgres=# select * from dblink('mydb', 'select * from test') as test(id integer, info varchar(8));

ERROR:  value too long for type character varying(8)

postgres=# select * from dblink('mydb', 'select * from test') as test(id integer, info varchar(32));

id |               info

----+----------------------------------

1 | c53bc2b29be0ef76d863a53efed13ea1

2 | 28fd974058eb778168482942c8894004

3 | 6e90dbdf605adffe70761aa8521ab626

4 | c32edb06e0f7e0067322f346ec60e663

5 | 2eb0c9fcb931ede7440244745c686177

6 | d34a41454758897e9b9d0c2dd1a972ae

7 | 09d8e45db5f32a20e4e77ed51cf34b1a

8 | 86cb5c36edda6fffd9f9c75aa9c3f0fe

9 | 7ae28a3b304291dbcb96d19b92b42012

10 | 581fddcce1c9e66a42ae4fad8a604e0d

(10 rows)

postgres=# select dblink_get_connections();

dblink_get_connections

------------------------

{mydb}

(1 row)

创建视图，来固化查询

postgres=# create view test_mydb as select * from dblink('mydb', 'select * from test') as test(id integer, info varchar(32));

CREATE VIEW

postgres=# select dblink_connect('mydb', 'dbname=mydb host=localhost');

dblink_connect

----------------

OK

(1 row)

postgres=# select * from test_mydb;

id |               info

----+----------------------------------

1 | c53bc2b29be0ef76d863a53efed13ea1

2 | 28fd974058eb778168482942c8894004

3 | 6e90dbdf605adffe70761aa8521ab626

4 | c32edb06e0f7e0067322f346ec60e663

5 | 2eb0c9fcb931ede7440244745c686177

6 | d34a41454758897e9b9d0c2dd1a972ae

7 | 09d8e45db5f32a20e4e77ed51cf34b1a

8 | 86cb5c36edda6fffd9f9c75aa9c3f0fe

9 | 7ae28a3b304291dbcb96d19b92b42012

10 | 581fddcce1c9e66a42ae4fad8a604e0d

(10 rows)

执行没有返回结果的SQL

postgres=# select dblink_exec('mydb', 'insert into test values(1, ''dblink insert i am'')');

dblink_exec

-------------

INSERT 0 1

(1 row)

通fetch的方式分页获取数据：

postgres=# select dblink_open('mydb', 'foo', 'select * from test');

dblink_open

-------------

OK

(1 row)

^

postgres=# select * from dblink_fetch('mydb', 'foo', 3) as test(id int, info text);

id |               info

----+----------------------------------

1 | c53bc2b29be0ef76d863a53efed13ea1

2 | 28fd974058eb778168482942c8894004

3 | 6e90dbdf605adffe70761aa8521ab626

(3 rows)

postgres=# select * from dblink_fetch('mydb', 'foo', 3) as (id int, info text);

id |               info

----+----------------------------------

4 | c32edb06e0f7e0067322f346ec60e663

5 | 2eb0c9fcb931ede7440244745c686177

6 | d34a41454758897e9b9d0c2dd1a972ae

(3 rows)

postgres=# (id int, info text);^C

postgres=# select * from dblink_fetch('mydb', 'foo', 3) as (id int, info text);

id |               info

----+----------------------------------

7 | 09d8e45db5f32a20e4e77ed51cf34b1a

8 | 86cb5c36edda6fffd9f9c75aa9c3f0fe

9 | 7ae28a3b304291dbcb96d19b92b42012

(3 rows)

postgres=# select * from dblink_fetch('mydb', 'foo', 3) as (id int, info text);

id |               info

----+----------------------------------

10 | 581fddcce1c9e66a42ae4fad8a604e0d

1 | dblink insert i am

(2 rows)

使用异步调用：dblink_send_query、dblink_is_busy、dblink_get_notify和dblink_get_result

postgres=# select * from dblink_send_query('mydb', 'select * from test;');

dblink_send_query

-------------------

1

(1 row)

postgres=# select * from dblink_get_result('mydb') as (id int, info text);

id |               info

----+----------------------------------

1 | c53bc2b29be0ef76d863a53efed13ea1

2 | 28fd974058eb778168482942c8894004

3 | 6e90dbdf605adffe70761aa8521ab626

4 | c32edb06e0f7e0067322f346ec60e663

5 | 2eb0c9fcb931ede7440244745c686177

6 | d34a41454758897e9b9d0c2dd1a972ae

7 | 09d8e45db5f32a20e4e77ed51cf34b1a

8 | 86cb5c36edda6fffd9f9c75aa9c3f0fe

9 | 7ae28a3b304291dbcb96d19b92b42012

10 | 581fddcce1c9e66a42ae4fad8a604e0d

1 | dblink insert i am

(11 rows)

postgres=# select * from dblink_send_query('mydb', 'select * from test;');

NOTICE:  could not send query: another command is already in progress

dblink_send_query

-------------------

0

(1 row)

—获取到了最后一条，再执行新的SQL才会成功

postgres=# select * from dblink_get_result('mydb') as (id int, info text);

id | info

----+------

(0 rows)

postgres=# select * from dblink_send_query('mydb', 'select * from test;');

dblink_send_query

-------------------

1

(1 row)

获取错误信息：

postgres=# select dblink_error_message('mydb');

dblink_error_message

----------------------

OK

(1 row)

能否用dblink去连接oracle数据库呢？像oracle的dblink一样，连接SQL Server、MySQL、PostgreSQL？

答案是不行的，源码里面可以看到PostgreSQL的dblink是使用的pg的c语言接口去创建连接的，而不是使用ODBC来创建：

dblink_connect(PG_FUNCTION_ARGS)

{

char *conname_or_str = NULL;

char *connstr = NULL;

char *connname = NULL;

char *msg;

PGconn *conn = NULL;

remoteConn *rconn = NULL;

dblink_init();

if (PG_NARGS() == 2)

{

conname_or_str = text_to_cstring(PG_GETARG_TEXT_PP(1));

connname = text_to_cstring(PG_GETARG_TEXT_PP(0));

}

else if (PG_NARGS() == 1)

conname_or_str = text_to_cstring(PG_GETARG_TEXT_PP(0));

if (connname)

rconn = (remoteConn *) MemoryContextAlloc(TopMemoryContext,

sizeof(remoteConn));

/* first check for valid foreign data server */

connstr = get_connect_string(conname_or_str);

if (connstr == NULL)

connstr = conname_or_str;

/* check password in connection string if not superuser */

dblink_connstr_check(connstr);

conn = PQconnectdb(connstr);

if (PQstatus(conn) == CONNECTION_BAD)

{

msg = pchomp(PQerrorMessage(conn));

PQfinish(conn);

if (rconn)

pfree(rconn);

ereport(ERROR,

(errcode(ERRCODE_SQLCLIENT_UNABLE_TO_ESTABLISH_SQLCONNECTION),

errmsg("could not establish connection"),

errdetail_internal("%s", msg)));

}

/* check password actually used if not superuser */

dblink_security_check(conn, rconn);

/* attempt to set client encoding to match server encoding, if needed */

if (PQclientEncoding(conn) != GetDatabaseEncoding())

PQsetClientEncoding(conn, GetDatabaseEncodingName());

if (connname)

{

rconn->conn = conn;

createNewConnection(connname, rconn);

}

else

{

if (pconn->conn)

PQfinish(pconn->conn);

pconn->conn = conn;

}

PG_RETURN_TEXT_P(cstring_to_text("OK"));

}