CREATE PUBLIC DATABASE LINK v10

Name

CREATE [PUBLIC] DATABASE LINK -- create a new database link.

Synopsis

CREATE [ PUBLIC ] DATABASE LINK <name>
  CONNECT TO { CURRENT_USER |
               <username> IDENTIFIED BY '<password>'}
  USING { postgres_fdw '<fdw_connection_string>' |
        [ oci ] '<oracle_connection_string>' }

Description

CREATE DATABASE LINK creates a new database link. A database link is an object that allows a reference to a table or view in a remote database within a DELETE, INSERT, SELECT or UPDATE command. A database link is referenced by appending @dblink to the table or view name referenced in the SQL command where dblink is the name of the database link.

Database links can be public or private. A public database link is one that can be used by any user. A private database link can be used only by the database link’s owner. Specification of the PUBLIC option creates a public database link. If omitted, a private database link is created.

When the CREATE DATABASE LINK command is given, the database link name and the given connection attributes are stored in the Advanced Server system table named, pg_catalog.edb_dblink. When using a given database link, the database containing the edb_dblink entry defining this database link is called the local database. The server and database whose connection attributes are defined within the edb_dblink entry is called the remote database. You can use edb_dblink_oci to access remote Oracle tables and views using any SELECT, INSERT, UPDATE, or DELETE statement.

A SQL command containing a reference to a database link must be issued while connected to the local database. When the SQL command is executed, the appropriate authentication and connection is made to the remote database to access the table or view to which the @dblink reference is appended.

Note

A database link cannot be used to access a remote database within a standby database server. Standby database servers are used for high availability, load balancing, and replication.

For information about high availability, load balancing, and replication for Postgres database servers, see the PostgreSQL core documentation available at:

https://www.postgresql.org/docs/10/static/high-availability.html

Note
  • For Advanced Server 10, the CREATE DATABASE LINK command is tested against and certified for use with Oracle version 10g Release 2 (10.2), Oracle version 11g Release 2 (11.2), and Oracle version 12c Release 1 (12.1).
  • The edb_dblink_oci.rescans GUC can be set to SCROLL or SERIALIZABLE at the server level in postgresql.conf file. It can also be set at session level using the SET command, but the setting will not be applied to existing dblink connections due to dblink connection caching.

The edb_dblink_oci supports both types of rescans: SCROLL and SERIALIZABLE. By default it is set to SERIALIZABLE. When set to SERIALIZABLE, edb_dblink_oci uses the SERIALIZABLE transaction isolation level on the Oracle side, which corresponds to PostgreSQL’s REPEATABLE READ.

  • This is necessary as a single PostgreSQL statement can lead to multiple Oracle queries and thereby uses a serializable isolation level to provide consistent results.
  • A serialization failure may occur due to a table modification concurrent with long-running DML transactions (for example ADD, UPDATE, or DELETE statements). If such a failure occurs, the OCI reports ORA-08177: can't serialize access for this transaction, and the application must retry the transaction.
  • A SCROLL rescan will be quick, but with each iteration will reset the current row position to 1. A SERIALIZABLE rescan has performance benefits over a SCROLL rescan.

Parameters

PUBLIC

Create a public database link that can be used by any user. If omitted, then the database link is private and can only be used by the database link’s owner.

name

The name of the database link.

username

The username to be used for connecting to the remote database.

CURRENT_USER

Include CURRENT_USER to specify that Advanced Server should use the user mapping associated with the role that is using the link when establishing a connection to the remote server.

password

The password for username.

postgres_fdw

Specifies foreign data wrapper postgres_fdw as the connection to a remote Advanced Server database. If postgres_fdw has not been installed on the database, use the CREATE EXTENSION command to install postgres_fdw. For more information, see the CREATE EXTENSION command in the PostgreSQL Core documentation at: https://www.postgresql.org/docs/10/static/sql-createextension.html

fdw_connection_string

Specify the connection information for the postgres_fdw foreign data wrapper.

oci

Specifies a connection to a remote Oracle database. This is Advanced Server’s default behavior.

oracle_connection_string

Specify the connection information for an oci connection.

Note

To create a non-public database link you must have the CREATE DATABASE LINK privilege. To create a public database link you must have the CREATE PUBLIC DATABASE LINK privilege.

In order to use oci-dblink, an Oracle instant client must be downloaded and installed on the host running the Advanced Server database in which the database link is to be created.

An instant client can be downloaded from the following site:

http://www.oracle.com/technetwork/database/features/instant-client/index-097480.html

Oracle Instant Client for Linux

The following instructions apply to Linux hosts running Advanced Server.

Be sure the libaio library (the Linux-native asynchronous I/O facility) has already been installed on the Linux host running Advanced Server.

The libaio library can be installed with the following command:

yum install libaio

If the Oracle instant client that you've downloaded does not include the file specifically named libclntsh.so without a version number suffix, you must create a symbolic link named libclntsh.so that points to the downloaded version of the library file. Navigate to the instant client directory and execute the following command:

ln -s libclntsh.so.version libclntsh.so

Where version is the version number of the libclntsh.so library. For example:

ln -s libclntsh.so.12.1 libclntsh.so

When you are executing a SQL command that references a database link to a remote Oracle database, Advanced Server must know where the Oracle instant client library resides on the Advanced Server host.

The LD_LIBRARY_PATH environment variable must include the path to the Oracle client installation directory containing the libclntsh.so file. For example, assuming the installation directory containing libclntsh.so is /tmp/instantclient:

export LD_LIBRARY_PATH=/tmp/instantclient:$LD_LIBRARY_PATH

Alternatively, you can also set the value of the oracle_home configuration parameter in the postgresql.conf file. So, the oracle_home configuration parameter is an alternative to the LD_LIBRARY_PATH environment variable. For more details on the oracle_home configuration parameter, see configuration paramters.

The ORACLE_HOME environment variable is must to be set and should include the path to the Oracle home directory. For example,

export ORACLE_HOME=/opt/product/version/dbhomeXE
Note

Either the LD_LIBRARY_PATH or the oracle_home configuration parameter must be set along with the ORACLE_HOME environment variable before the pg_ctl utility is executed to start or restart Advanced Server.

If you are running the current session as the user account (for example, enterprisedb) that will directly invoke pg_ctl to start or restart Advanced Server, then be sure to set either the LD_LIBRARY_PATH environment variable or the oracle_home configuration parameter and the ORACLE_HOME environment variable before invoking pg_ctl.

You can set the LD_LIBRARY_PATH and the ORACLE_HOME environment variable within the .bash_profile file under the home directory of the enterprisedb user account (that is, set LD_LIBRARY_PATH and ORACLE_HOME within file ~enterprisedb/.bash_profile). This ensures that LD_LIBRARY_PATH and ORACLE_HOME will be set when you log in as enterprisedb.

If you are using a Linux service script with the systemctl or service command to start or restart Advanced Server, you must set LD_LIBRARY_PATH and ORACLE_HOME so it is in effect when the script invokes the pg_ctl utility.

For example, to set an environment variable for Advanced Server, you can create a file named /etc/systemd/system/edb-as-10.service; include /lib/systemd/system/edb-as-10.service within the file.

Assuming the LD_LIBRARY_PATH=/tmp/instantclient you can now include the environment variable by specifying:

[Service]
Environment=LD_LIBRARY_PATH=/tmp/instantclient:$LD_LIBRARY_PATH
Environment=ORACLE_HOME=/tmp/instantclient

Then, use the following command to reload systemd:

systemctl daemon-reload

Then, restart the Advanced Server service with the following command:

systemctl restart edb-as-10

The particular script file that needs to be modified to include the LD_LIBRARY_PATH setting depends upon the Advanced Server version, the Linux system on which it was installed, and whether it was installed with the graphical installer or an RPM package.

See the appropriate version of the EDB Postgres Advanced Server Installation Guide to determine the service script that affects the startup environment. The installation guides can be found at the following location:

EDB docs website

Oracle Instant Client for Windows

The following instructions apply to Windows hosts running Advanced Server.

When you are executing a SQL command that references a database link to a remote Oracle database, Advanced Server must know where the Oracle instant client library resides on the Advanced Server host.

Set the Windows PATH system environment variable to include the Oracle client installation directory that contains the oci.dll file.

As an alternative you, can set the value of the oracle_home configuration parameter in the postgresql.conf file. The value specified in the oracle_home configuration parameter will override the Windows PATH environment variable.

To set the oracle_home configuration parameter in the postgresql.conf file, edit the file, adding the following line:

oracle_home = 'lib_directory'

Substitute the name of the Windows directory that contains oci.dll for lib_directory. For example:

oracle_home = 'C:/tmp/instantclient_10_2'

After setting the PATH environment variable or the oracle_home configuration parameter, you must restart the server for the changes to take effect. Restart the server from the Windows Services console.

Examples

The following example demonstrates using the CREATE DATABASE LINK command to create a database link (named chicago) that connects an instance of Advanced Server to an Oracle server via an oci-dblink connection. The connection information tells Advanced Server to log in to Oracle as user admin, whose password is mypassword. Including the oci option tells Advanced Server that this is an oci-dblink connection; the connection string, '//127.0.0.1/acctg' specifies the server address and name of the database.

CREATE DATABASE LINK chicago
  CONNECT TO admin IDENTIFIED BY 'mypassword'
  USING oci '//127.0.0.1/acctg';
Note

You can specify a hostname in the connection string (in place of an IP address).

The following example demonstrates using the CREATE DATABASE LINK command to create a database link (named bedford) that connects an instance of Advanced Server to another Advanced Server instance via a postgres_fdw foreign data wrapper connection. The connection information tells Advanced Server to log in as user admin, whose password is mypassword. Including the postgres_fdw option tells Advanced Server that this is a postgres_fdw connection; the connection string, 'host=127.0.0.1 port=5444 dbname=marketing' specifies the server address and name of the database.

CREATE DATABASE LINK bedford
  CONNECT TO admin IDENTIFIED BY 'mypassword'
  USING postgres_fdw 'host=127.0.0.1 port=5444 dbname=marketing';
Note

You can specify a hostname in the connection string (in place of an IP address).

The following examples demonstrate using a database link with Advanced Server to connect to an Oracle database. The examples assume that a copy of the Advanced Server sample application’s emp table has been created in an Oracle database and a second Advanced Server database cluster with the sample application is accepting connections at port 5443.

Create a public database link named, oralink, to an Oracle database named, xe, located at 127.0.0.1 on port 1521. Connect to the Oracle database with username, edb, and password, password.

CREATE PUBLIC DATABASE LINK oralink CONNECT TO edb IDENTIFIED BY 'password'
USING '//127.0.0.1:1521/xe';

Issue a SELECT command on the emp table in the Oracle database using database link, oralink.

SELECT * FROM emp@oralink;

 empno | ename  |    job    | mgr  |      hiredate      | sal  | comm | deptno
-------+--------+-----------+------+--------------------+------+------+--------
  7369 | SMITH  | CLERK     | 7902 | 17-DEC-80 00:00:00 |  800 |      |     20
  7499 | ALLEN  | SALESMAN  | 7698 | 20-FEB-81 00:00:00 | 1600 |  300 |     30
  7521 | WARD   | SALESMAN  | 7698 | 22-FEB-81 00:00:00 | 1250 |  500 |     30
  7566 | JONES  | MANAGER   | 7839 | 02-APR-81 00:00:00 | 2975 |      |     20
  7654 | MARTIN | SALESMAN  | 7698 | 28-SEP-81 00:00:00 | 1250 | 1400 |     30
  7698 | BLAKE  | MANAGER   | 7839 | 01-MAY-81 00:00:00 | 2850 |      |     30
  7782 | CLARK  | MANAGER   | 7839 | 09-JUN-81 00:00:00 | 2450 |      |     10
  7788 | SCOTT  | ANALYST   | 7566 | 19-APR-87 00:00:00 | 3000 |      |     20
  7839 | KING   | PRESIDENT |      | 17-NOV-81 00:00:00 | 5000 |      |     10
  7844 | TURNER | SALESMAN  | 7698 | 08-SEP-81 00:00:00 | 1500 |    0 |     30
  7876 | ADAMS  | CLERK     | 7788 | 23-MAY-87 00:00:00 | 1100 |      |     20
  7900 | JAMES  | CLERK     | 7698 | 03-DEC-81 00:00:00 |  950 |      |     30
  7902 | FORD   | ANALYST   | 7566 | 03-DEC-81 00:00:00 | 3000 |      |     20
  7934 | MILLER | CLERK     | 7782 | 23-JAN-82 00:00:00 | 1300 |      |     10
(14 rows)

Create a private database link named, fdwlink, to the Advanced Server database named, edb, located on host 192.168.2.22 running on port 5444. Connect to the Advanced Server database with username, enterprisedb, and password, password.

CREATE DATABASE LINK fdwlink CONNECT TO enterprisedb IDENTIFIED BY
'password' USING postgres_fdw 'host=192.168.2.22 port=5444 dbname=edb';

Display attributes of database links, oralink and fdwlink, from the local edb_dblink system table:

SELECT lnkname, lnkuser, lnkconnstr FROM pg_catalog.edb_dblink;

 lnkname |   lnkuser    |               lnkconnstr
---------+--------------+----------------------------------------
 oralink | edb          | //127.0.0.1:1521/xe
 fdwlink | enterprisedb |
(2 rows)

Perform a join of the emp table from the Oracle database with the dept table from the Advanced Server database:

SELECT d.deptno, d.dname, e.empno, e.ename, e.job, e.sal, e.comm FROM
emp@oralink e, dept@fdwlink d WHERE e.deptno = d.deptno ORDER BY 1, 3;

 deptno |   dname    | empno | ename  |    job    | sal  | comm
--------+------------+-------+--------+-----------+------+------
     10 | ACCOUNTING |  7782 | CLARK  | MANAGER   | 2450 |
     10 | ACCOUNTING |  7839 | KING   | PRESIDENT | 5000 |
     10 | ACCOUNTING |  7934 | MILLER | CLERK     | 1300 |
     20 | RESEARCH   |  7369 | SMITH  | CLERK     |  800 |
     20 | RESEARCH   |  7566 | JONES  | MANAGER   | 2975 |
     20 | RESEARCH   |  7788 | SCOTT  | ANALYST   | 3000 |
     20 | RESEARCH   |  7876 | ADAMS  | CLERK     | 1100 |
     20 | RESEARCH   |  7902 | FORD   | ANALYST   | 3000 |
     30 | SALES      |  7499 | ALLEN  | SALESMAN  | 1600 |  300
     30 | SALES      |  7521 | WARD   | SALESMAN  | 1250 |  500
     30 | SALES      |  7654 | MARTIN | SALESMAN  | 1250 | 1400
     30 | SALES      |  7698 | BLAKE  | MANAGER   | 2850 |
     30 | SALES      |  7844 | TURNER | SALESMAN  | 1500 |    0
     30 | SALES      |  7900 | JAMES  | CLERK     |  950 |
(14 rows)

When the oci-dblink is used to execute SQL statements on a remote Oracle database, there are certain circumstances where pushdown of the processing occurs on the foreign server.

Pushdown refers to the occurrence of processing on the foreign (that is, the remote) server instead of the local client where the SQL statement was issued. Pushdown can result in performance improvement since the data is processed on the remote server before being returned to the local client.

Pushdown applies to statements with the standard SQL join operations (inner join, left outer join, right outer join, and full outer join). Pushdown still occurs even when a sort is specified on the resulting data set.

In order for pushdown to occur, certain basic conditions must be met. The tables involved in the join operation must belong to the same foreign server and use the identical connection information to the foreign server (that is, the same database link defined with the CREATE DATABASE LINK command).

In order to determine if pushdown is to be used for a SQL statement, display the execution plan by using the EXPLAIN command.

For information about the EXPLAIN command, see the PostgreSQL Core documentation at:

https://www.postgresql.org/docs/10/static/sql-explain.html

The following examples use the database link created as shown by the following:

CREATE PUBLIC DATABASE LINK oralink CONNECT TO edb IDENTIFIED BY 'password'
USING '//192.168.2.23:1521/xe';

The following example shows the execution plan of an inner join:

EXPLAIN (verbose,costs off) SELECT d.deptno, d.dname, e.empno, e.ename FROM
dept@oralink d, emp@oralink e WHERE d.deptno = e.deptno ORDER BY 1, 3;

                                        QUERY PLAN
--------------------------------------------------------------------------------
 Foreign Scan
   Output: d.deptno, d.dname, e.empno, e.ename
   Relations: (_dblink_dept_1 d) INNER JOIN (_dblink_emp_2 e)
   Remote Query: SELECT r1.deptno, r1.dname, r2.empno, r2.ename FROM (dept r1 INNER 
JOIN emp r2 ON ((r1.deptno = r2.deptno))) ORDER BY r1.deptno ASC NULLS LAST, r2.empno ASC NULLS LAST
(4 rows)

Note that the INNER JOIN operation occurs under the Foreign Scan section. The output of this join is the following:

deptno |   dname    | empno | ename
-------+------------+-------+--------
    10 | ACCOUNTING |  7782 | CLARK
    10 | ACCOUNTING |  7839 | KING
    10 | ACCOUNTING |  7934 | MILLER
    20 | RESEARCH   |  7369 | SMITH
    20 | RESEARCH   |  7566 | JONES
    20 | RESEARCH   |  7788 | SCOTT
    20 | RESEARCH   |  7876 | ADAMS
    20 | RESEARCH   |  7902 | FORD
    30 | SALES      |  7499 | ALLEN
    30 | SALES      |  7521 | WARD
    30 | SALES      |  7654 | MARTIN
    30 | SALES      |  7698 | BLAKE
    30 | SALES      |  7844 | TURNER
    30 | SALES      |  7900 | JAMES
(14 rows)

The following shows the execution plan of a left outer join:

EXPLAIN (verbose,costs off) SELECT d.deptno, d.dname, e.empno, e.ename FROM
dept@oralink d LEFT OUTER JOIN emp@oralink e ON d.deptno = e.deptno ORDER BY 1, 3;

                                        QUERY PLAN
--------------------------------------------------------------------------------
 Foreign Scan
   Output: d.deptno, d.dname, e.empno, e.ename
   Relations: (_dblink_dept_1 d) LEFT JOIN (_dblink_emp_2 e)
   Remote Query: SELECT r1.deptno, r1.dname, r2.empno, r2.ename FROM (dept r1 LEFT JOIN 
emp r2 ON ((r1.deptno = r2.deptno))) ORDER BY r1.deptno ASC NULLS LAST, r2.empno ASC NULLS LAST
(4 rows)

The output of this join is the following:

deptno |   dname    | empno | ename
-------+------------+-------+--------
    10 | ACCOUNTING |  7782 | CLARK
    10 | ACCOUNTING |  7839 | KING
    10 | ACCOUNTING |  7934 | MILLER
    20 | RESEARCH   |  7369 | SMITH
    20 | RESEARCH   |  7566 | JONES
    20 | RESEARCH   |  7788 | SCOTT
    20 | RESEARCH   |  7876 | ADAMS
    20 | RESEARCH   |  7902 | FORD
    30 | SALES      |  7499 | ALLEN
    30 | SALES      |  7521 | WARD
    30 | SALES      |  7654 | MARTIN
    30 | SALES      |  7698 | BLAKE
    30 | SALES      |  7844 | TURNER
    30 | SALES      |  7900 | JAMES
    40 | OPERATIONS |       |
(15 rows)

The following example shows a case where the entire processing is not pushed down because the emp joined table resides locally instead of on the same foreign server.

EXPLAIN (verbose,costs off) SELECT d.deptno, d.dname, e.empno, e.ename FROM
dept@oralink d LEFT OUTER JOIN emp e ON d.deptno = e.deptno ORDER BY 1, 3;

                            QUERY PLAN
------------------------------------------------------------------
 Sort
   Output: d.deptno, d.dname, e.empno, e.ename
   Sort Key: d.deptno, e.empno
   ->  Hash Left Join
         Output: d.deptno, d.dname, e.empno, e.ename
         Hash Cond: (d.deptno = e.deptno)
         ->  Foreign Scan on _dblink_dept_1 d
               Output: d.deptno, d.dname, d.loc
               Remote Query: SELECT deptno, dname, NULL FROM dept
         ->  Hash
               Output: e.empno, e.ename, e.deptno
               ->  Seq Scan on public.emp e
                     Output: e.empno, e.ename, e.deptno
(13 rows)

The output of this join is the same as the previous left outer join example.

Note

The procedure described in this section is not compatible with Oracle databases.

After you have created a database link, you can create a foreign table based upon this database link. The foreign table can then be used to access the remote table referencing it with the foreign table name instead of using the database link syntax. Using the database link requires appending @dblink to the table or view name referenced in the SQL command where dblink is the name of the database link.

This technique can be used for either an oci-dblink connection for remote Oracle access, or a postgres_fdw connection for remote Postgres access.

The following example shows the creation of a foreign table to access a remote Oracle table.

First, create a database link as previously described. The following is the creation of a database link named oralink for connecting to the Oracle database.

CREATE PUBLIC DATABASE LINK oralink CONNECT TO edb IDENTIFIED BY 'password'
USING '//127.0.0.1:1521/xe';

The following query shows the database link:

SELECT lnkname, lnkuser, lnkconnstr FROM pg_catalog.edb_dblink;

 lnkname | lnkuser |     lnkconnstr
---------+---------+---------------------
 oralink | edb     | //127.0.0.1:1521/xe
(1 row)

When you create the database link, Advanced Server creates a corresponding foreign server. The following query displays the foreign server:

SELECT srvname, srvowner, srvfdw, srvtype, srvoptions FROM
pg_foreign_server;

 srvname | srvowner | srvfdw | srvtype |          srvoptions
---------+----------+--------+---------+-------------------------------
 oralink |       10 |  14005 |         | {connstr=//127.0.0.1:1521/xe}
(1 row)

For more information about foreign servers, see the CREATE SERVER command in the PostgreSQL Core documentation at:

https://www.postgresql.org/docs/10/static/sql-createserver.html

Create the foreign table as shown by the following:

CREATE FOREIGN TABLE emp_ora (
    empno           NUMERIC(4),
    ename           VARCHAR(10),
    job             VARCHAR(9),
    mgr             NUMERIC(4),
    hiredate        TIMESTAMP WITHOUT TIME ZONE,
    sal             NUMERIC(7,2),
    comm            NUMERIC(7,2),
    deptno          NUMERIC(2)
)
  SERVER oralink
  OPTIONS (table_name 'emp', schema_name 'edb'
);

Note the following in the CREATE FOREIGN TABLE command:

  • The name specified in the SERVER clause at the end of the CREATE FOREIGN TABLE command is the name of the foreign server, which is oralink in this example as displayed in the srvname column from the query on pg_foreign_server.
  • The table name and schema name are specified in the OPTIONS clause by the table and schema options.
  • The column names specified in the CREATE FOREIGN TABLE command must match the column names in the remote table.
  • Generally, CONSTRAINT clauses may not be accepted or enforced on the foreign table as they are assumed to have been defined on the remote table.

For more information about the CREATE FOREIGN TABLE command, see the PostgreSQL Core documentation at:

https://www.postgresql.org/docs/10/static/sql-createforeigntable.html

The following is a query on the foreign table:

SELECT * FROM emp_ora;

 empno | ename  |    job    | mgr  |      hiredate      |   sal   |  comm   | deptno
-------+--------+-----------+------+--------------------+---------+---------+--------
  7369 | SMITH  | CLERK     | 7902 | 17-DEC-80 00:00:00 |  800.00 |         |    20
  7499 | ALLEN  | SALESMAN  | 7698 | 20-FEB-81 00:00:00 | 1600.00 |  300.00 |    30
  7521 | WARD   | SALESMAN  | 7698 | 22-FEB-81 00:00:00 | 1250.00 |  500.00 |    30
  7566 | JONES  | MANAGER   | 7839 | 02-APR-81 00:00:00 | 2975.00 |         |    20
  7654 | MARTIN | SALESMAN  | 7698 | 28-SEP-81 00:00:00 | 1250.00 | 1400.00 |    30
  7698 | BLAKE  | MANAGER   | 7839 | 01-MAY-81 00:00:00 | 2850.00 |         |    30
  7782 | CLARK  | MANAGER   | 7839 | 09-JUN-81 00:00:00 | 2450.00 |         |    10
  7788 | SCOTT  | ANALYST   | 7566 | 19-APR-87 00:00:00 | 3000.00 |         |    20
  7839 | KING   | PRESIDENT |      | 17-NOV-81 00:00:00 | 5000.00 |         |    10
  7844 | TURNER | SALESMAN  | 7698 | 08-SEP-81 00:00:00 | 1500.00 |    0.00 |    30
  7876 | ADAMS  | CLERK     | 7788 | 23-MAY-87 00:00:00 | 1100.00 |         |    20
  7900 | JAMES  | CLERK     | 7698 | 03-DEC-81 00:00:00 |  950.00 |         |    30
  7902 | FORD   | ANALYST   | 7566 | 03-DEC-81 00:00:00 | 3000.00 |         |    20
  7934 | MILLER | CLERK     | 7782 | 23-JAN-82 00:00:00 | 1300.00 |         |    10
(14 rows)

In contrast, the following is a query on the same remote table, but using the database link instead of the foreign table:

SELECT * FROM emp@oralink;

 empno | ename  |    job    | mgr  |      hiredate      | sal  | comm | deptno
-------+--------+-----------+------+--------------------+------+------+--------
  7369 | SMITH  | CLERK     | 7902 | 17-DEC-80 00:00:00 |  800 |      |     20
  7499 | ALLEN  | SALESMAN  | 7698 | 20-FEB-81 00:00:00 | 1600 |  300 |     30
  7521 | WARD   | SALESMAN  | 7698 | 22-FEB-81 00:00:00 | 1250 |  500 |     30
  7566 | JONES  | MANAGER   | 7839 | 02-APR-81 00:00:00 | 2975 |      |     20
  7654 | MARTIN | SALESMAN  | 7698 | 28-SEP-81 00:00:00 | 1250 | 1400 |     30
  7698 | BLAKE  | MANAGER   | 7839 | 01-MAY-81 00:00:00 | 2850 |      |     30
  7782 | CLARK  | MANAGER   | 7839 | 09-JUN-81 00:00:00 | 2450 |      |     10
  7788 | SCOTT  | ANALYST   | 7566 | 19-APR-87 00:00:00 | 3000 |      |     20
  7839 | KING   | PRESIDENT |      | 17-NOV-81 00:00:00 | 5000 |      |     10
  7844 | TURNER | SALESMAN  | 7698 | 08-SEP-81 00:00:00 | 1500 |    0 |     30
  7876 | ADAMS  | CLERK     | 7788 | 23-MAY-87 00:00:00 | 1100 |      |     20
  7900 | JAMES  | CLERK     | 7698 | 03-DEC-81 00:00:00 |  950 |      |     30
  7902 | FORD   | ANALYST   | 7566 | 03-DEC-81 00:00:00 | 3000 |      |     20
  7934 | MILLER | CLERK     | 7782 | 23-JAN-82 00:00:00 | 1300 |      |     10
(14 rows)
Note

For backward compatibility reasons, it is still possible to write USING libpq rather than USING postgres_fdw. However, the libpq connector is missing many important optimizations which are present in the postgres_fdw connector. Therefore, the postgres_fdw connector should be used whenever possible. The libpq option is deprecated and may be removed entirely in a future Advanced Server release.

See also

DROP DATABASE LINK