PostgreSQL dblink使用过程
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"));
}
