Ways to access Oracle Database in PostgreSQL

Today, organizations stores information(data) in different database systems. Each database system has a set of applications that run against it. This data is just bits and bytes on a file system - and only a database can turn the bits and bytes of data into business information. Integration and consolidation of such information(data) into one database system is often difficult. Because many of the applications that run against one database may not have an equivalent application that runs against another. To consolidate the information into one database system, we need a heterogeneous database connection.  In this post, I'll demo on how you may connect PostgreSQL to one of heterogeneous database Oracle using different methods.

Below are few methods to make connection to Oracle database in PostgreSQL.

• Using ODBC Driver
• Using Foreign DataWrappers
• Using Oracle Call Interface(OCI) Driver

Softwares used in demo(included download links) - CentOS 7 64bit, PostgreSQL 9.5, EDB Postgres Advanced Server 9.5, ODBC-Link 1.0.4, unixODBC-2.3.4, Oracle Instant Client 11.x Drivers & Oracle_FDW 

Using ODBC Driver

Open DataBase Connectivity(ODBC) is a standard software API for using DBMS. The ODBC driver/ODBC Data source(API) is a library that allows applications to connect to any database for which an ODBC driver is available. It's a middle layer translates the application's data queries into commands that the DBMS understands. To use this method, an open source unixODBC and Oracle ODBC driver(Basic/ODBC/Devel) packages required. Along with a module in PostgreSQL that can communicate to DSN created using unixODBC and Oracle ODBC driver. Few years back CyberTec has released a module ODBC-Link, at present it is obsolete, however, it has a dblink-style implementation for PostgreSQL to connect to any other ODBC compliant database. We can use this module for basic connectivity to Oracle. Let's see.

Install unixODBC

tar -xvf unixODBC-2.3.4.tar.gz
cd unixODBC-2.3.4/
./configure --sysconfdir=/etc
make
make install

Binary/Libraries/configuration files location: /usr/local/bin,/usr/local/lib,/etc(odbc.ini,odbcinst.ini)

Install Oracle ODBC Driver

rpm -ivh oracle-instantclient11.2-basic-11.2.0.4.0-1.x86_64.rpm
rpm -ivh oracle-instantclient11.2-odbc-11.2.0.4.0-1.x86_64.rpm
rpm -ivh oracle-instantclient11.2-devel-11.2.0.4.0-1.x86_64.rpm

Binary/Libraries location: /usr/lib/oracle/11.2/client64

Install ODBC-Link

tar -zxvf ODBC-Link-1.0.4.tar.gz
cd ODBC-Link-1.0.4
export PATH=/opt/PostgreSQL/9.5/bin:$PATH
which pg_config 
make USE_PGXS=1
make USE_PGXS=1 install

Libraries and SQL files location: /opt/PostgreSQL/9.5/share/postgresql/contrib

Installation will create a ODBC-Link module SQL file in $PGHOME/contrib directory. Load the SQL file, which will create a schema by name "odbclink" with necessary functions in it.

psql -p 5432 -d oratest -U postgres -f /opt/PostgreSQL/9.5/share/postgresql/contrib/odbclink.sql

At this point, we have installed unixODBC Drirver, Oracle ODBC driver and ODBC-Link module for PostgreSQL. As a first step, we need to create a DSN using Oracle ODBC.

Edit /etc/odbcinst.ini file and pass the drivers deifinition

## Driver for Oracle
[MyOracle]
Description     =ODBC for oracle
Driver          =/usr/lib/oracle/11.2/client64/lib/libsqora.so.11.1
UsageCount=1
FileUsage = 1
Driver Logging = 7

Edit /etc/odbc.ini file and create the DSN with driver mentioned in /etd/odbcinst.ini

## Host: pg.raghav-node1.com, PORT: 1521
## Oracle Instance Name: ORA11G, Username: mmruser, Password: mmruser
## ODBC Data source: Ora

[Ora]
Description = myoracledb database
Driver = MyOracle
Trace = yes
TraceFile = /tmp/odbc_oracle.log
Database = //pg.raghav-node1.com:1521/ORA11G
UserID = mmruser
Password = mmruser
Port = 1521

After creating DSN, load all Oracle & unix ODBC driver libraries by setting environment variables and test the connectivity using OS command line tool "dltest" & "iSQL"

[root@172.16.210.161 ~]# export ORACLE_HOME=/usr/lib/oracle/11.2/client64
[root@172.16.210.161 ~]# export LD_LIBRARY_PATH=/usr/local/unixODBC-2.3.4/lib:/usr/lib/oracle/11.2/client64/lib
[root@172.16.210.161 ~]# export ODBCINI=/etc/odbc.ini
[root@172.16.210.161 ~]# export ODBCSYSINI=/etc/
[root@172.16.210.161 ~]# export TWO_TASK=//pg.raghav-node1.com:1521/ORA11G
[root@172.16.210.161 ~]# dltest /usr/lib/oracle/11.2/client64/lib/libsqora.so.11.1
SUCCESS: Loaded /usr/lib/oracle/11.2/client64/lib/libsqora.so.11.1
[root@172.16.210.161 ~]# isql ora -v
+---------------------------------------+
| Connected!                            |
|                                       |
| sql-statement                         |
| help [tablename]                      |
| quit                                  |
|                                       |
+---------------------------------------+
SQL>

Now, set the same environment variables for postgres user for loading the libraries and restart the PostgreSQL cluster to take effect. Connect to PostgreSQL and call odbclink functions to connect to Oracle database.

[root@172.16.210.163 ~]#su - postgres
[postgres@172.16.210.163 ~]$ export ORACLE_HOME=/usr/lib/oracle/11.2/client64
[postgres@172.16.210.163 ~]$ export LD_LIBRARY_PATH=/usr/local/unixODBC-2.3.4/lib:/usr/lib/oracle/11.2/client64/lib
[postgres@172.16.210.163 ~]$ export ODBCINI=/etc/odbc.ini
[postgres@172.16.210.163 ~]$ export ODBCSYSINI=/etc/
[postgres@172.16.210.163 ~]$ export TWO_TASK=//pg.raghav-node1.com:1521/ORA11G
[postgres@172.16.210.163 ~]$ dltest /usr/lib/oracle/11.2/client64/lib/libsqora.so.11.1
SUCCESS: Loaded /usr/lib/oracle/11.2/client64/lib/libsqora.so.11.1
[postgres@172.16.210.163 ~]$ /opt/PostgreSQL/9.5/bin/pg_ctl -D /opt/PostgreSQL/9.5/data/ stop -mf
[postgres@172.16.210.163 ~]$ /opt/PostgreSQL/9.5/bin/pg_ctl -D /opt/PostgreSQL/9.5/data/ start
[postgres@172.16.210.163 ~]$ psql
psql.bin (9.5.2)
Type "help" for help.

postgres=# select odbclink.connect('DSN=Ora');
 connect
---------
       1
(1 row)

Cool right...!!!. For retrieving and manipulating data refer to ODBC-Link README file.

Using Foreign DataWrappers

An SQL/MED(SQL Management of External Data) extension to the SQL Standard allows managing external data stored outside the database. SQL/MED provides two components Foreign data wrappers and Datalink. PostgreSQL introduced Foreign Data Wrapper(FDW) in 9.1 version with read-only support and in 9.3 version write support of this SQL Standard. Today, the latest version has a number of features around it and many varieties of FDW available to access different remote SQL databases.

Oracle_fdw provides an easy and efficient way to access Oracle Database. IMO,its one of the coolest method to access the remote database. To compile Oracle_FDW with PostgreSQL 9.5, we need Oracle Instant Client libraries and pg_config set in PATH. We can use the same Oracle Instant Client libraries used for ODBC-Link. Let's see how it works.

First, set environment variables with OIC libraries and pg_config

export PATH=/opt/PostgreSQL/9.5/bin:$PATH
export ORACLE_HOME=/usr/lib/oracle/11.2/client64
export LD_LIBRARY_PATH=/usr/lib/oracle/11.2/client64/lib

Unzip the oracle_fdw module and compile it with PostgreSQL 9.5

unzip oracle_fdw-1.4.0.zip
cd oracle_fdw-1.4.0/
make 
make install

Now switch as 'postgres' user and restart the cluster by loading Oracle Instant Client libraries required for oracle_fdw extension and create the extension inside the database.

[postgres@172.16.210.161 9.5]$ export ORACLE_HOME=/usr/lib/oracle/11.2/client64/lib
[postgres@172.16.210.161 9.5]$ export LD_LIBRARY_PATH=/usr/lib/oracle/11.2/client64/lib:$LD_LIBRARY_PATH
[postgres@172.16.210.161 9.5]$ /opt/PostgreSQL/9.5/bin/pg_ctl -D /opt/PostgreSQL/9.5/data/ stop -mf
[postgres@172.16.210.161 9.5]$ /opt/PostgreSQL/9.5/bin/pg_ctl -D /opt/PostgreSQL/9.5/data/ start
[postgres@172.16.210.161 9.5]$ psql
Password:
psql.bin (9.5.2)
Type "help" for help.

postgres=# create extension oracle_fdw;
CREATE EXTENSION

Now you can access the Oracle database.

postgres=# CREATE SERVER oradb FOREIGN DATA WRAPPER oracle_fdw OPTIONS (dbserver '//pg.raghav-node1.com/ORA11G');
CREATE SERVER
postgres=# GRANT USAGE ON FOREIGN SERVER oradb TO postgres;
GRANT
postgres=# CREATE USER MAPPING FOR postgres SERVER oradb OPTIONS (user 'scott', password 'tiger');
CREATE USER MAPPING
postgres=#
postgres=# CREATE FOREIGN TABLE oratab (ecode integer,name char(30)) SERVER oradb OPTIONS(schema 'SCOTT',table 'EMP');
CREATE FOREIGN TABLE
postgres=# select * from oratab limit 3;
 ecode |              name
-------+--------------------------------
  7369 | SMITH
  7499 | ALLEN
  7521 | WARD
(3 rows)

Using Oracle Call Interface(OCI) Drivers

Oracle Call Interface(OCI) a type-2 driver freely available on Oracle site which allows the client to connect to Oracle database. EDB Postgres Advanced Server (also called EPAS) a proprietary product has built-in OCI-based database link module called dblink_ora, which connects to Oracle database using Oracle OCI drivers. All you have to do to use dblink_ora module, install EPAS(not covering installation) and tell EPAS where it can find Oracle OCI driver libraries. We can make use of same Oracle Instant Client by specifying its libraries location in LD_LIBRARY_PATH environment variable and to take effect restart the EPAS cluster.

First, switch as "enterprisedb" user, load the libraries and restart the cluster. That's all, we are good to access Oracle database.

[enterprisedb@172.16.210.129 ~]$ export LD_LIBRARY_PATH=/usr/lib/oracle/11.2/client64/lib
[enterprisedb@172.16.210.129 bin]$ /opt/PostgresPlus/9.5AS/bin/pg_ctl -D /opt/PostgresPlus/9.5AS/data/ restart
[enterprisedb@172.16.210.129 bin]$ psql
psql.bin (9.5.0.5)
Type "help" for help.

edb=# select dblink_ora_connect('oraconn','localhost','edbora','edbuser','edbuser',1521);
 dblink_ora_connect
--------------------
 OK
(1 row)

Note: EPAS connects to the Oracle Database using Oracle Instant Client library "libclntsh.so". If you won't find the library in Oracle Client Library location then create the symbolic link with libclntsh.so pointing to the libclntsh.so.version.number. Refer to documentation.

In the example, dblink_ora_connect establishes a connection to an Oracle database with the user-specified connection information. Later using link name('oraconn' in my case) we can perform operations like SELECT,INSERT,DELETE,UPDATE & COPY using dblink_ora* functions. All functions you can refer from the EnterpriseDB documentation here.

All the above methods will be very handy if you are working on migration projects. Hope its helpful. Thank you

--Raghav