Foreign Data Wrapper for Oracle
oracle_fdw is a PostgreSQL extension that provides a Foreign Data Wrapper for
easy and efficient access to Oracle databases, including pushdown of WHERE
conditions and required columns as well as comprehensive EXPLAIN support.
oracle_fdw was written by Laurenz Albe <firstname.lastname@example.org>
This README contains the following sections:
2. Objects created by the extension
5. Installation Requirements
This is a simple example how to use oracle_fdw.
More detailed information will be provided in the sections "Options" and
"Usage" below. You should also read the PostgreSQL documentation on
foreign data and the commands used in the example.
For the sake of this example, let's assume you can connect as operating system
user "postgres" (or whoever starts the PostgreSQL server) with the following
That means that the Oracle client and the environment is set up correctly.
I also assume that oracle_fdw has been compiled and installed (see section
We want to access a table defined like this:
SQL> DESCRIBE oratab
Name Null? Type
------------------------------- -------- ------------
ID NOT NULL NUMBER(5)
FLOATING NOT NULL NUMBER(7,2)
Then configure oracle_fdw as PostgreSQL superuser like this:
pgdb=# CREATE EXTENSION oracle_fdw;
pgdb=# CREATE SERVER oradb FOREIGN DATA WRAPPER oracle_fdw
OPTIONS (dbserver '//dbserver.mydomain.com/ORADB');
pgdb=# GRANT USAGE ON FOREIGN SERVER oradb TO pguser;
(You can use other naming methods or local connections, see the description of
option "dbserver" below.)
Then you can connect to PostgreSQL as "pguser" and define:
pgdb=> CREATE USER MAPPING FOR pguser SERVER oradb
OPTIONS (user 'orauser', password 'orapwd');
(You can use external authentication to avoid storing Oracle passwords;
pgdb=> CREATE FOREIGN TABLE oratab (
id integer NOT NULL,
text character varying(30),
floating double precision NOT NULL
) SERVER oradb OPTIONS (schema 'ORAUSER', table 'ORATAB');
(Remember that table and schema name -- the latter is optional -- must
normally be in uppercase.)
Now you can use the table like a regular PostgreSQL table.
2. Objects created by the extension
FUNCTION oracle_fdw_handler() RETURNS fdw_handler
Oracle foreign data wrapper handler
FUNCTION oracle_fdw_validator(text, oid) RETURNS void
Oracle foreign data wrapper options validator
These functions are the handler and the validator function necessary to create
a foreign data wrapper.
The extension automatically creates a foreign data wrapper named "oracle_fdw".
Normally that's all you need and you can proceed to define foreign servers.
You can create additional Oracle foreign data wrappers, for example if you
need to set the "nls_lang" option (you can alter the existing "oracle_fdw"
wrapper, but all modifications will be lost after a dump/restore).
FUNCTION oracle_close_connections() RETURNS void
closes all open Oracle connections
This function can be used to close all open Oracle connections in this session.
See "Usage" below for when this might be useful.
Foreign data wrapper options
(Caution: If you modify the default foreign data wrapper "oracle_fdw",
any changes will be lost upon dump/restore. Create a new foreign data wrapper
if you want the options to be persistent. The SQL script shipped with the
software contains a CREATE FOREIGN DATA WRAPPER statement you can use.)
- nls_lang (optional)
Sets the NLS_LANG environment variable for Oracle to this value.
NLS_LANG is in the form <language>_<territory>.<charset> (for example
AMERICAN_AMERICA.AL32UTF8). This must match your database encoding.
When this value is not set, oracle_fdw will automatically do the right
thing if it can and issue a warning if it cannot.
Set this only if you know what you are doing. See "Problems" below.
Foreign server options
- dbserver (required)
The Oracle database connection string for the remote database.
This can be in any of the forms that Oracle supports as long as your
Oracle client is configured accordingly.
Set this to an empty string for local ("BEQUEATH") connections.
User mapping options
- user (required)
The Oracle user name for the session.
Set this to an empty string for "external authentication" if you don't
want to store Oracle credentials in the PostgreSQL database (one simple way
is to use an "external password store").
- password (required)
The password for the Oracle user.
Foreign table options
- table (required)
The Oracle table name. This name must be written exactly as it occurs in
Oracle's system catalog, so normally consist of uppercase letters only.
- schema (optional)
The table's schema (or owner). Useful to access tables that do not belong
to the connecting Oracle user. This name must be written exactly as it
occurs in Oracle's system catalog, so normally consist of uppercase letters
- plan_costs (optional, defaults to "off")
If set to yes/on/true, Oracle's cost estimates will be used. The problem is
that Oracle gives good estimates for the result size, but not for the cost
of the execution (the only estimate you can get is "execution time" in second
Since this cost estimate is almost useless but expensive to collect,
oracle_fdw by default does not bother to get Oracle's cost estimates and
estimates the cost to 10000 regardless of the actual query.
From PostgreSQL 9.2 on, it is usually better to gather statistics on the
foreign table (see ANALYZE below) and keep this option disabled.
Turn this on only if a) query execution is expensive and b) it has a positive
influence on PostgreSQL query planning.
- max_long (optional, defaults to "32767")
The maximal length of any LONG or LONG RAW columns in the Oracle table.
Possible values are integers between 1 and 1073741823 (the maximal size of a
bytea in PostgreSQL). This amount of memory will be allocated at least
twice, so large values will consume a lot of memory.
If "max_long" is less than the length of the longest value retrieved,
you will receive the error message "ORA-01406: fetched column value was
- readonly (optional, defaults to "false")
INSERT, UPDATE and DELETE is only allowed on tables where this option is
not set to yes/on/true. Since these statements can only be executed from
PostgreSQL 9.3 on, setting this option has no effect on earlier versions.
It might still be a good idea to set it in PostgreSQL 9.2 and earlier
on tables that you do not wish to be changed, to be prepared for an upgrade
to PostgreSQL 9.3 or later.
Column options (from PostgreSQL 9.2 on)
- key (optional, defaults to "false")
If set to yes/on/true, the corresponding column on the foreign Oracle table
is considered a primary key column.
For UPDATE and DELETE to work, you must set this option on all columns
that belong to the table's primary key.
The Oracle user will obviously need CREATE SESSION privilege and the right
to select from the table or view in question.
For EXPLAIN VERBOSE (and query planning if "plan_costs" is turned on),
the user will also need SELECT privileges on V$SQL and V$SQL_PLAN.
oracle_fdw caches Oracle connections because it is expensive to create an
Oracle session for each individual query. All connections are automatically
closed when the PostgreSQL session ends.
The function oracle_close_connections() can be used to close all cached Oracle
connections. This can be useful for long-running sessions that don't access
foreign tables all the time and want to avoid blocking the resources needed
by an open Oracle connection.
Don't call this function inside a transaction that modifies Oracle data.
When you define a foreign table, the columns of the Oracle table are mapped
to the PostgreSQL columns in the order of their definition.
oracle_fdw will only include those columns in the Oracle query that are
actually needed by the PostgreSQL query.
The PostgreSQL table can have more or less columns than the Oracle table.
If it has more columns, and these columns are used, you will receive a warning
and NULL values will be returned.
If you want to UPDATE or DELETE, make sure that the "key" option is set on all
columns that belong to the table's primary key. Failure to do so will result
You must define the PostgreSQL columns with data types that oracle_fdw can
translate (see the conversion table below). This restriction is only enforced
if the column actually gets used, so you can define "dummy" columns for
untranslatable data types as long as you don't access them (this trick only
works with SELECT, not when modifying foreign data). If an Oracle value
exceeds the size of the PostgreSQL column (e.g., the length of a varchar
column or the maximal integer value), you will receive a runtime error.
These conversions are automatically handled by oracle_fdw:
Oracle type | Possible PostgreSQL types
CHAR | char, varchar, text
NCHAR | char, varchar, text
VARCHAR | char, varchar, text
VARCHAR2 | char, varchar, text
NVARCHAR2 | char, varchar, text
CLOB | char, varchar, text
LONG | char, varchar, text
RAW | uuid, bytea
BLOB | bytea
BFILE | bytea (read-only)
LONG RAW | bytea
NUMBER | numeric, float4, float8, char, varchar, text
NUMBER(n,m) with m<=0 | numeric, float4, float8, int2, int4, int8,
| boolean, char, varchar, text
FLOAT | numeric, float4, float8, char, varchar, text
BINARY_FLOAT | numeric, float4, float8, char, varchar, text
BINARY_DOUBLE | numeric, float4, float8, char, varchar, text
DATE | date, timestamp, timestamptz, char,
| varchar, text
TIMESTAMP | date, timestamp, timestamptz, char,
| varchar, text
TIMESTAMP WITH TIME ZOME | date, timestamp, timestamptz, char,
| varchar, text
TIMESTAMP WITH LOCAL TIME ZOME | date, timestamp, timestamptz, char,
| varchar, text
INTERVAL YEAR TO MONTH | interval, char, varchar, text
INTERVAL DAY TO SECOND | interval, char, varchar, text
If a NUMBER is converted to a boolean, "0" means "false", everything else "true".
NCLOB is currently not supported because Oracle cannot automatically convert
it to the client encoding.
If you need conversions exceeding the above, define an appropriate view in
Oracle or PostgreSQL.
PostgreSQL will use all applicaple parts of the WHERE clause as a filter
for the scan. The Oracle query that oracle_fdw constructs will contain a WHERE
clause corresponding to these filter criteria whenever such a condition can
safely be translated to Oracle SQL. This feature, also known as "push-down
of WHERE clauses", can greatly reduce the number of rows retrieved from Oracle
and may enable Oracle's optimizer to choose a good plan for accessing the
To make use of that, try to use simple conditions for the foreign table.
Choose PostgreSQL column data types that correspond to Oracle's types,
because otherwise conditions cannot be translated.
The expressions "now()", "transaction_timestamp()", "current_timestamp",
"current_date" and "localtimestamp" will be translated correctly.
The output of EXPLAIN will show the Oracle query used, so you can see which
conditions were translated to Oracle and how.
Modifying foreign data
From PostgreSQL 9.3 on, oracle_fdw supports INSERT, UPDATE and DELETE on
foreign tables. This is allowed by default (also in databases upgraded
from an earlier PostgreSQL release) and can be disabled by setting the
"readonly" table option.
For UPDATE and DELETE to work, the columns corresponding to the primary
key columns of the Oracle table must have the "key" column option set.
These columns are used to identify a foreign table row, so make sure that
the option is set on *all* columns that belong to the primary key.
If you omit a foreign table column during INSERT, that column is set to
the value defined in the DEFAULT clause on the PostgreSQL foreign table
(or NULL if there is no DEFAULT clause). DEFAULT clauses on the
corresponding Oracle columns are not used.
If the PostgreSQL foreign table does not include all columns of the
Oracle table, the Oracle DEFAULT clauses will be used for the columns not
included in the foreign table definition.
The RETURNING clause on INSERT, UPDATE and DELETE is supported except
for columns with Oracle data types LONG and LONG RAW (Oracle doesn't support
these data types in the RETURNING clause).
While modifying foreign data works, the performance is not particularly
good, specifically when many rows are affected, because (owing to the way
foreign data wrappers work) each row has to be treated individually.
Transactions are forwarded to Oracle, so BEGIN, COMMIT, ROLLBACK and
SAVEPOINT work as expected. Prepared statements involving Oracle are
not supported. See "Internals" for details.
Since oracle_fdw uses serialized transactions, it is possible that
data modifying statements lead to a serialization failure:
ORA-08177: can't serialize access for this transaction
This can happen if concurrent transactions modify the table and gets more
likely in long running transactions. Such errors can be identified by their
SQLSTATE (40001). An application using oracle_fdw should retry transactions
that fail with this error.
PostgreSQL's EXPLAIN will show the query that is actually issued to Oracle.
EXPLAIN VERBOSE will show Oracle's execution plan.
From PostgreSQL version 9.2 on, you can use ANALYZE to gather statistics
on a foreign table. This is supported by oracle_fdw.
Statistics will be used for query planning, and for many queries will
result in good (and much faster) row count estimates even if the table option
"plan_costs" is turned off as long as the statistics are accurate.
Keep in mind that analyzing an Oracle foreign table will result in a full
sequential table scan.
5. Installation Requirements
oracle_fdw should compile and run on any platform supported by PostgreSQL and
Oracle client, although I could only test it on Linux and Windows.
PostgreSQL 9.1 or better is required.
Support for ANALYZE is available from PostgreSQL 9.2 on.
Support for INSERT, UPDATE and DELETE is available from PostgreSQL 9.3 on.
Oracle client version 10.1 or better is required.
oracle_fdw can be built and used with Oracle Instant Client as well as with
Oracle Client and Server installations installed with Universal Installer.
Binaries compiled with Oracle Client 10 can be used with later client versions
without recompilation or relink.
The supported Oracle server versions depend on the used client version (see the
Oracle Client/Server Interoperability Matrix in support document 207303.1).
For maximum coverage use Oracle Client 10.2.0.5, as this will allow you to
connect to every server version from 8.1.7 to 12.1.0 except 9.0.1.
It is advisable to use the latest Patch Set on both Oracle client and server,
particularly with desupported Oracle versions.
For a list of Oracle bugs that are known to affect oracle_fdw's usability,
see the "Problems" section below.
oracle_fdw has been written as a PostgreSQL extension and uses the Extension
Building Infrastructure "PGXS". It should be easy to install.
You will need PostgreSQL headers and PGXS installed (if your PostgreSQL was
installed with packages, install the development package).
You need to install Oracle's C header files as well (SDK package for Instant
Make sure that PostgreSQL is configured "--without-ldap" (at least the server).
See "Problems" below.
Make sure that "pg_config" is in the PATH (test with "pg_config --pgxs").
Set the environment variable ORACLE_HOME to the location of the Oracle
Unpack the source code of oracle_fdw and change into the directory.
Then the software installation should be as simple as
$ make install
For the second step you need write permission to PostgreSQL's shared library
Since the Oracle client shared library is probably not in the standard
library path, you have to make sure that the PostgreSQL server will be able
to find it. How this is done varies from operating system to operating
system; on Linux you can set LD_LIBRARY_PATH or use /etc/ld.so.conf.
Make sure that all necessary Oracle environment variables are set in the
environment of the PostgreSQL server process (ORACLE_HOME if you don't use
Instant Client, TNS_ADMIN if you have configuration files, etc.)
To install the extension in a database, connect as superuser and
CREATE EXTENSION oracle_fdw;
That will define the required functions and create a foreign data wrapper.
oracle_fdw sets the MODULE of the Oracle session to "postgres" and the
ACTION to the backend process number. This can help identifying the Oracle
session and allows you to trace it with DBMS_MONITOR.SERV_MOD_ACT_TRACE_ENABLE.
oracle_fdw uses Oracle's result prefetching to avoid unnecessary client-server
round-trips. The prefetch counter is set to 200, the memory limit to 24KB.
These limits can be changed in oracle_utils.c.
Rather than using a PLAN_TABLE to explain an Oracle query (which would require
such a table to be created in the Oracle database), oracle_fdw uses execution
plans stored in the library cache. For that, an Oracle query is "explicitly
described", which forces Oracle to parse the query. The hard part is to find
the SQL_ID and CHILD_NUMBER of the statement in V$SQL because the SQL_TEXT
column contains only the first 1000 bytes of the query.
Therefore, oracle_fdw adds a comment to the query that contains an MD5 hash
of the query text. This is used to search in V$SQL.
The actual execution plan or cost information is retrieved from V$SQL_PLAN.
oracle_fdw uses transaction isolation level "serializable" on the Oracle side,
which corresponds to PostgreSQL's repeatable read. This is necessary because
a single PostgreSQL statement can lead to multiple Oracle queries (e.g. during
a nested loop join) and the results need to be consistent.
The Oracle transaction is committed immediately before the local transaction
commits, so that a completed PostgreSQL transaction guarantees that the Oracle
transaction has completed. However, there is a small chance that the
PostgreSQL transaction cannot complete even though the Oracle transaction
is committed. This cannot be avoided without using two-phase transactions
and a transaction manager, which is clearly beyond what a foreign data wrapper
can reasonably provide.
Prepared statements involving Oracle are not supported for the same reason.
Characters stored in an Oracle database that cannot be converted to the
PostgreSQL database encoding will silently be replaced by "replacement
characters", typically a normal or inverted question mark, by Oracle.
You will get no warning or error messages.
If you use a PostgreSQL database encoding that Oracle does not know
(currently, these are EUC_CN, EUC_KR, LATIN10, MULE_INTERNAL, WIN874
and SQL_ASCII), non-ASCII characters cannot be translated correctly.
You will get a warning in this case, and the characters will be replaced
by replacement characters as described above.
You can set the "nls_lang" option of the foreign data wrapper to force a
certain Oracle encoding, but the resulting characters will most likely be
incorrect and lead to PostgreSQL error messages. This is probably only
useful for SQL_ASCII encoding if you know what you are doing.
See "Options" above.
Oracle's planner does not give good cost estimates (you can only get the
estimated execution time in second granularity, and it is never less than
one). So by default, oracle_fdw does not use these estimates.
See the description of the "plan_costs" option above.
Even without that, the current implementation of oracle_fdw requires
calls to the Oracle server for every foreign table during query planning.
That means that there might be a noticeable performance improvement
if query plans are cached, for example by using prepared statements or
The Oracle client shared library comes with its own LDAP client
implementation conforming to RFC 1823, so these functions have the same
names as OpenLDAP's. This will lead to a name collision when the PostgreSQL
server was configured "--with-ldap".
The name collision will not be detected, because oracle_fdw is loaded at
runtime, but trouble will happen if anybody calls an LDAP function.
Typically, OpenLDAP is loaded first, so if Oracle calls an LDAP function
(for example if you use "directory naming" name resolution), the backend
will crash. This can lead to messages like the following (seen on Linux)
in the PostgreSQL server log:
Assertion `( (ld)->ld_options.ldo_valid == 0x2 )' failed.
The best thing is to configure PostgreSQL "--without-ldap". This is the only
safe way to avoid this problem.
Even when PostgreSQL is built "--with-ldap", it may work as long as you don't
use any LDAP client functionality in Oracle.
On some platforms, you can force Oracle's client shared library to be loaded
before the PostgreSQL server is started (LD_PRELOAD on Linux). Then Oracle's
LDAP functions should get used. In that case, Oracle may be able to use
LDAP functionality, but using LDAP from PostgreSQL will crash the backend.
You cannot use LDAP functionality both in PostgreSQL and in Oracle, period.
In Oracle 11.2 or above, inserting the first row into a newly created
Oracle table with oracle_fdw will lead to a serialization error.
This is because of an Oracle feature called "deferred segment creation" which
defers allocation of storage space for a new table until the first row
is inserted. This causes a serialization failure with serializable
transactions (see document 1285464.1 in Oracle's knowledge base).
This is no serious problem; you can work around it by either ignoring that
first error or creating the table with "SEGMENT CREATION IMMEDIATE".
This is a list of Oracle bugs that have affected oracle_fdw in the past.
Bug 6039623 can cause oracle_fdw to crash with a segmentation violation
when larger amounts of data are selected from an Oracle table via an Oracle
It can occur with any Oracle server version below 12.1, and the only remedy
is to install patch 10096945 which is included in the 184.108.40.206 Patch Set.
Bug 2728408 can cause "ORA-8177 cannot serialize access for this transaction"
even if no modification of remote data is attempted.
It can occur with Oracle server 220.127.116.11 (install one-off patch 2728408) or
Oracle server 9.2 (install Patch Set 18.104.22.168 or better).
If you have a problem or question, the preferred option is to send an
e-mail to email@example.com
You can subscribe here:
There is a mail archive here:
There is the option to open an issue on GitHub:
This requires a GitHub account.
You can also use the issue trackers on pgFoundry:
pgFoundry also offers forums for questions:
The trackers and forums require a pgFoundry account.
I will also answer e-mail sent to me at firstname.lastname@example.org, but that way
you exclude others from helping you or benefiting from your experience.