dblink

The dblink module supports connections to other Greenplum Database databases from within a database session. These databases can reside in the same Greenplum Database system, or in a remote system.

Greenplum Database supports dblink connections between databases in Greenplum Database installations with the same major version number. You can also use dblink to connect to other Greenplum Database installations that use compatible libpq libraries.

Note
dblink is intended for database users to perform short ad hoc queries in other databases. dblink is not intended for use as a replacement for external tables or for administrative tools such as gpcopy.

The Greenplum Database dblink module is a modified version of the PostgreSQL dblink module. There are some restrictions and limitations when you use the module in Greenplum Database.

Installing and Registering the Module

The dblink module is installed when you install Greenplum Database. Before you can use any of the functions defined in the module, you must register the dblink extension in each database in which you want to use the functions. Refer to Installing Additional Supplied Modules for more information.

Greenplum Database Considerations

In this release of Greenplum Database, statements that modify table data cannot use named or implicit dblink connections. Instead, you must provide the connection string directly in the dblink() function. For example:

gpadmin=# CREATE TABLE testdbllocal (a int, b text) DISTRIBUTED BY (a);
CREATE TABLE
gpadmin=# INSERT INTO testdbllocal select * FROM dblink('dbname=postgres', 'SELECT * FROM testdblink') AS dbltab(id int, product text);
INSERT 0 2

The Greenplum Database version of dblink deactivates the following asynchronous functions:

dblink_send_query()
dblink_is_busy()
dblink_get_result()

Using dblink

The following procedure identifies the basic steps for configuring and using dblink in Greenplum Database. The examples use dblink_connect() to create a connection to a database and dblink() to run an SQL query.

Begin by creating a sample table to query using the dblink functions. These commands create a small table in the postgres database, which you will later query from the testdb database using dblink:

$ psql -d postgres
psql (9.4.20)
Type "help" for help.

postgres=# CREATE TABLE testdblink (a int, b text) DISTRIBUTED BY (a);
CREATE TABLE
postgres=# INSERT INTO testdblink VALUES (1, 'Cheese'), (2, 'Fish');
INSERT 0 2
postgres=# \q
$

Log into a different database as a superuser. In this example, the superuser gpadmin logs into the database testdb. If the dblink functions are not already available, register the dblink extension in the database:
```
$ psql -d testdb
psql (9.4beta1)
Type "help" for help.

testdb=# CREATE EXTENSION dblink;
CREATE EXTENSION
```
Use the dblink_connect() function to create either an implicit or a named connection to another database. The connection string that you provide should be a libpq-style keyword/value string. This example creates a connection named mylocalconn to the postgres database on the local Greenplum Database system:
```
testdb=# SELECT dblink_connect('mylocalconn', 'dbname=postgres user=gpadmin');
 dblink_connect
----------------
 OK
(1 row)
```
Note
If a user is not specified, dblink_connect() uses the value of the PGUSER environment variable when Greenplum Database was started. If PGUSER is not set, the default is the system user that started Greenplum Database.
Use the dblink() function to query a database using a configured connection. Keep in mind that this function returns a record type, so you must assign the columns returned in the dblink() query. For example, the following command uses the named connection to query the table you created earlier:
```
testdb=# SELECT * FROM dblink('mylocalconn', 'SELECT * FROM testdblink') AS dbltab(id int, product text);
 id | product
----+---------
  1 | Cheese
  2 | Fish
(2 rows)
```

To connect to the local database as another user, specify the user in the connection string. This example connects to the database as the user test_user. Using dblink_connect(), a superuser can create a connection to another local database without specifying a password.

testdb=# SELECT dblink_connect('localconn2', 'dbname=postgres user=test_user');

To make a connection to a remote database system, include host and password information in the connection string. For example, to create an implicit dblink connection to a remote system:

testdb=# SELECT dblink_connect('host=remotehost port=5432 dbname=postgres user=gpadmin password=secret');

Using dblink as a Non-Superuser

To make a connection to a database with dblink_connect(), non-superusers must include host, user, and password information in the connection string. The host, user, and password information must be included even when connecting to a local database. You must also include an entry in pg_hba.conf for this non-superuser and the target database. For example, the user test_user can create a dblink connection to the local system mdw with this command:

testdb=> SELECT dblink_connect('host=mdw port=5432 dbname=postgres user=test_user password=secret');

If non-superusers need to create dblink connections that do not require a password, they can use the dblink_connect_u() function. The dblink_connect_u() function is identical to dblink_connect(), except that it allows non-superusers to create connections that do not require a password.

dblink_connect_u() is initially installed with all privileges revoked from PUBLIC, making it un-callable except by superusers. In some situations, it may be appropriate to grant EXECUTE permission on dblink_connect_u() to specific users who are considered trustworthy, but this should be done with care.

Caution
If a Greenplum Database system has configured users with an authentication method that does not involve a password, then impersonation and subsequent escalation of privileges can occur when a non-superuser runs dblink_connect_u(). The dblink connection will appear to have originated from the user specified by the function. For example, a non-superuser can run dblink_connect_u() and specify a user that is configured with trust authentication.

Also, even if the dblink connection requires a password, it is possible for the password to be supplied from the server environment, such as a ~/.pgpass file belonging to the server's user. It is recommended that any ~/.pgpass file belonging to the server's user not contain any records specifying a wildcard host name.

As a superuser, grant the EXECUTE privilege on the dblink_connect_u() functions in the user database. This example grants the privilege to the non-superuser test_user on the functions with the signatures for creating an implicit or a named dblink connection. The server and database will be identified through a standard libpq connection string and optionally, a name can be assigned to the connection.
```
testdb=# GRANT EXECUTE ON FUNCTION dblink_connect_u(text) TO test_user;
testdb=# GRANT EXECUTE ON FUNCTION dblink_connect_u(text, text) TO test_user;
```
Now test_user can create a connection to another local database without a password. For example, test_user can log into the testdb database and run this command to create a connection named testconn to the local postgres database.
```
testdb=> SELECT dblink_connect_u('testconn', 'dbname=postgres user=test_user');
```
Note
If a user is not specified, dblink_connect_u() uses the value of the PGUSER environment variable when Greenplum Database was started. If PGUSER is not set, the default is the system user that started Greenplum Database.
test_user can use the dblink() function to run a query using a dblink connection. For example, this command uses the dblink connection named testconn created in the previous step. test_user must have appropriate access to the table.
```
testdb=> SELECT * FROM dblink('testconn', 'SELECT * FROM testdblink') AS dbltab(id int, product text);
```

Using dblink as a Non-Superuser without Authentication Checks

In rare cases you may need to allow non-superusers to acccess to dblink without making any authentication checks. The function dblink_connect_no_auth() provides this functionality as it bypasses the pg_hba.conf file.

Caution
Using this function introduces a security risk; ensure that you grant unauthorized access only to trusted user accounts. Also note that dblink_connect_no_auth() functions limit connections to the local cluster, and do not permit connections to a remote database.

These functions are not available by default; the gpadmin superuser must grant permission to the non-superuser beforehand:

As a superuser, grant the EXECUTE privilege on the dblink_connect_no_auth() functions in the user database. This example grants the privilege to the non-superuser test_user on the functions with the signatures for creating an implicit or a named dblink connection.
```
testdb=# GRANT EXECUTE ON FUNCTION dblink_connect_no_auth(text) TO test_user;
testdb=# GRANT EXECUTE ON FUNCTION dblink_connect_no_auth(text, text) TO test_user;
```
Now test_user can create a connection to another local database without providing a password, regardless of what is specified in pg_hba.conf. For example, test_user can log into the testdb database and execute this command to create a connection named testconn to the local postgres database.
```
testdb=> SELECT dblink_connect_no_auth('testconn', 'dbname=postgres user=test_user');
```
test_user can use the dblink() function to execute a query using a dblink connection. For example, this command uses the dblink connection named testconn created in the previous step. test_user must have appropriate access to the table.
```
testdb=> SELECT * FROM dblink('testconn', 'SELECT * FROM testdblink') AS dbltab(id int, product text);
```

Using dblink with SSL-Encrypted Connections to Greenplum

When you use dblink to connect to Greenplum Database over an encrypted connection, you must specify the sslmode property in the connection string. Set sslmode to at least require to disallow unencrypted transfers. For example:

testdb=# SELECT dblink_connect('greenplum_con_sales', 'dbname=sales host=gpmaster user=gpadmin sslmode=require');

Refer to SSL Client Authentication for information about configuring Greenplum Database to use SSL.

Additional Module Documentation

Refer to the dblink PostgreSQL documentation for detailed information about the individual functions in this module.