dblink Functions
A newer version of this documentation is available. Use the version menu above to view the most up-to-date release of the Greenplum 5.x documentation.
dblink Functions
The dblink module is provided for making easy connections to other databases either on the same database host, or on a remote host.
dblink is intended for database users to perform short ad hoc queries in other databases. dblink is not intended as a replacement for external tables or for administrative tools such as gptransfer.
The following procedure shows the basic steps for configuring and using dblink in Greenplum Database. See dblink in the PostgreSQL documentation for more information about individual functions.
Note: You must specify both a hostname and a password to connect with dblink as
a non-superuser.
- 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 gpadmin database using
dblink:
$ psql -d postgres psql (8.3.23) Type "help" for help. postgres=# CREATE TABLE testdblink (a int, b text) DISTRIBUTED BY (a); CREATE TABLE postgres=# INSERT INTO testdblink VALUES (1, 'Cheese'); INSERT 0 1 postgres=# INSERT INTO testdblink VALUES (2, 'Fish'); INSERT 0 1 postgres=# \q $
- Log into a different database (gpadmin in this example) and install the
dblink functions if they are not already available. You install the
dblink functions using the
$GPHOME/share/postgresql/contrib/dblink.sql
script:
$ psql -d gpadmin psql (8.3.23) Type "help" for help. gpadmin=# \i /usr/local/gpdb/share/postgresql/contrib/dblink.sql SET CREATE FUNCTION CREATE FUNCTION CREATE FUNCTION CREATE FUNCTION REVOKE REVOKE CREATE FUNCTION CREATE FUNCTION ...
- Use the dblink_connect function to create both implicit and named
connections to other databases. The connection string that you provide should be a
libpq-style keyword/value string. For example, to create a named connection to the
postgres database on the local Greenplum Database
system:
gpadmin=# SELECT dblink_connect('mylocalconn', 'dbname=postgres'); dblink_connect ---------------- OK (1 row)
To make a connection to a remote database system, simply include host and port information in the connection string. For example, to create an implicit dblink connection to a remote system:gpadmin=# SELECT dblink_connect('host=remotehost port=5432 dbname=postgres');
Note: You must specify both a hostname and a password in the connection string to connect as a non-superuser. - Use the basic dblink function to query a database using a configured
connection. Keep in mind that the dblink 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 in Step
1:
gpadmin=# SELECT * FROM dblink('mylocalconn', 'SELECT * FROM testdblink') AS dbltab(id int, product text); id | product ----+--------- 1 | Cheese 2 | Fish (2 rows)
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