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.

This README contains the following sections:

1. Cookbook
2. Objects created by the extension
3. Options
4. Usage
5. Installation Requirements
6. Installation
7. Internals
8. Problems
9. Support

oracle_fdw was written by Laurenz Albe, with notable contributions from Vincent Mora of Oslandia and Tatsuro Yamada of the NTT OSS Center.

Special thanks to Christian Ullrich for ongoing help with Windows.

1 Cookbook

This is a simple example how to use oracle_fdw.
More detailed information will be provided in the sections Options and Usage. You should also read the [PostgreSQL documentation on foreign data] (https://www.postgresql.org/docs/current/static/ddl-foreign-data.html) and the commands referenced there.

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 command:

sqlplus orauser/orapwd@//dbserver.mydomain.com:1521/ORADB

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 the Installation section).

We want to access a table defined like this:

SQL> DESCRIBE oratab
 Name                            Null?    Type
 ------------------------------- -------- ------------
 ID                              NOT NULL NUMBER(5)
 TEXT                                     VARCHAR2(30)
 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:1521/ORADB');
pgdb=# GRANT USAGE ON FOREIGN SERVER oradb TO pguser;

(You can use other naming methods or local connections, see the description of the 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; see below.)

pgdb=> CREATE FOREIGN TABLE oratab (
          id        integer           OPTIONS (key 'true')  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
FUNCTION oracle_fdw_validator(text[], oid) RETURNS void

These functions are the handler and the validator function necessary to create a foreign data wrapper.

FOREIGN DATA WRAPPER oracle_fdw
  HANDLER oracle_fdw_handler
  VALIDATOR oracle_fdw_validator

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

This function can be used to close all open Oracle connections in this session. See the Usage section for further description.

FUNCTION oracle_diag(name DEFAULT NULL) RETURNS text

This function is useful for diagnostic purposes only.
It will return the versions of oracle_fdw, PostgreSQL server and Oracle client. If called with no argument or NULL, it will additionally return the values of some environment variables used for establishing Oracle connections.
If called with the name of a foreign server, it will additionally return the Oracle server version.

3 Options

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 the Problems section.

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.

  To define a foreign table based on an arbitrary Oracle query, set this option to the query enclosed in parentheses, e.g.

  OPTIONS (table '(SELECT col FROM tab WHERE val = ''string'')')
  

  Do not set the schema option in this case.
  INSERT, UPDATE and DELETE will work on foreign tables defined on simple queries; if you want to avoid that (or confusing Oracle error messages for more complicated queries), use the table option readonly.

• 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 only.

• 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 truncated.

• 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.

• sample_percent (optional, defaults to "100")

  This option only influences ANALYZE processing and can be useful to ANALYZE very large tables in a reasonable time.

  The value must be between 0.000001 and 100 and defines the percentage of Oracle table blocks that will be randomly selected to calculate PostgreSQL table statistics. This is accomplished using the SAMPLE BLOCK (x) clause in Oracle.

  ANALYZE will fail with ORA-00933 for tables defined with Oracle queries and may fail with ORA-01446 for tables defined with complex Oracle views.

• prefetch (optional, defaults to "200")

  Sets the number of rows that will be fetched with a single round-trip between PostgreSQL and Oracle during a foreign table scan. This is implemented using Oracle row prefetching. The value must be between 0 and 10240, where a value of zero disables prefetching.

  Higher values can speed up performance, but will use more memory on the PostgreSQL server.

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.

4 Usage

Oracle permissions

The Oracle user will obviously need CREATE SESSION privilege and the right to select from the table or view in question.

For EXPLAIN VERBOSE the user will also need SELECT privileges on V$SQL and V$SQL_PLAN.

Connections

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.
You cannot call this function inside a transaction that modifies Oracle data.

Columns

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 in errors.

Data types

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, json
NVARCHAR2                | char, varchar, text
CLOB                     | char, varchar, text, json
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 ZONE | date, timestamp, timestamptz, char, varchar, text
TIMESTAMP WITH           | date, timestamp, timestamptz, char, varchar, text
   LOCAL TIME ZONE       |
INTERVAL YEAR TO MONTH   | interval, char, varchar, text
INTERVAL DAY TO SECOND   | interval, char, varchar, text
MDSYS.SDO_GEOMETRY       | geometry (see "PostGIS support" below)

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.

WHERE conditions and ORDER BY clauses

PostgreSQL will use all applicable 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 required tables.

Similarly, ORDER BY clauses will be pushed down to Oracle from PostgreSQL 9.2 on wherever possible. Note that no ORDER BY condition that sorts by a character string will be pushed down as the sort orders in PostgreSQL an Oracle cannot be guaranteed to be the same.

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.

Joins between foreign tables

From PostgreSQL 9.6 on, oracle_fdw can push down joins to the Oracle server, that is, a join between two foreign tables will lead to a single Oracle query that performs the join on the Oracle side.

There are some restrictions when this can happen:

• Both tables must be defined on the same foreign server.
• The join must be an inner join.
• Joins between three or more tables won't be pushed down.
• The join must be in a SELECT statement.
• oracle_fdw must be able to push down all join conditions and WHERE clauses.
• Cross joins without join conditions are not pushed down.
• If a join is pushed down, ORDER BY clauses will not be pushed down.

It is important that table statistics for both foreign tables have been collected with ANALYZE for PostgreSQL to determine the best join strategy.

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).

Triggers on foreign tables are supported from PostgreSQL 9.4. Triggers defined with AFTER and FOR EACH ROW require that the foreign table has no columns with Oracle data type LONG or LONG RAW. This is because such triggers make use of the RETURNING clause mentioned above.

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 the Internals section 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.

EXPLAIN

PostgreSQL's EXPLAIN will show the query that is actually issued to Oracle. EXPLAIN VERBOSE will show Oracle's execution plan (that will not work with Oracle server 9i or older, see Problems).

ANALYZE

From PostgreSQL version 9.2 on, you can use ANALYZE to gather statistics on a foreign table. This is supported by oracle_fdw.

Without statistics, PostgreSQL has no way to estimate the row count for queries on a foreign table, which can cause bad execution plans to be chosen.

PostgreSQL will not automatically gather statistics for foreign tables with the autovacuum daemon like it does for normal tables, so it is particularly important to run ANALYZE on foreign tables after creation and whenever the remote table has changed significantly.

Keep in mind that analyzing an Oracle foreign table will result in a full sequential table scan. You can use the table option sample_percent to speed this up by using only a sample of the Oracle table.

PostGIS support

The data type geometry is only available when PostGIS is installed.

The only supported geometry types are POINT, LINE, POLYGON, MULTIPOINT, MULTILINE and MULTIPOLYGON in two and three dimensions. Empty PostGIS geometries are not supported because they have no equivalent in Oracle Spatial.

NULL values for Oracle SRID will be converted to 0 and vice versa. For other conversions between Oracle SRID and PostGIS SRID, create a file srid.map in the PostgreSQL share directory. Each line of this file shall contain an Oracle SRID and the corresponding PostGIS SRID, separated by whitespace. Keep the file small for good performance.

Support for IMPORT FOREIGN SCHEMA

From PostgreSQL 9.5 on, IMPORT FOREIGN SCHEMA is supported to bulk import table definitions for all tables in an Oracle schema.
In addition to the documentation of IMPORT FOREIGN SCHEMA, consider the following:

• IMPORT FOREIGN SCHEMA will create foreign tables for all objects found in ALL_TAB_COLUMNS. That includes tables, views and materialized views, but not synonyms.

• There are two supported options for IMPORT FOREIGN SCHEMA:

  • case: controls case folding for table and column names during import.
    The possible values are:
    • keep: leave the names as they are in Oracle, usually in upper case.
    • lower: translate all table and column names to lower case.
    • smart: only translate names that are all upper case in Oracle (this is the default).
  • readonly (boolean): controls if imported tables can be modified.
    If set to true, all imported tables are created with the foreign table option readonly set to true (see the Options section).
    The default is false.

• The Oracle schema name must be written exactly as it is in Oracle, so normally in upper case. Since PostgreSQL translates names to lower case before processing, you must protect the schema name with double quotes (for example "SCOTT").

• Table names in the LIMIT TO or EXCEPT clause must be written as they will appear in PostgreSQL after the case folding described above.

Note that IMPORT FOREIGN SCHEMA does not work with Oracle server 8i; see the Problems section for details.

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.
PostgreSQL and Oracle need to have the same architecture, for example you cannot have 32-bit software for the one and 64-bit software for the other.

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.
Consult the oracle_fdw Wiki (https://github.com/laurenz/oracle_fdw/wiki) for tips about Oracle installation and configuration and share your own knowledge there.

6 Installation

If you are using a binary distribution of oracle_fdw, skip to "Installing the extension" below.

Building oracle_fdw on platforms other than Windows:

oracle_fdw has been written as a PostgreSQL extension and uses the Extension Building Infrastructure PGXS on all platforms except Windows. It should be easy to install. For building on Windows, see the next section.

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 Client). If you use the Instant Client ZIP files provided by Oracle, you will have to create a symbolic link from libclntsh.so to the actual shared library file yourself.

Make sure that PostgreSQL is configured --without-ldap (at least the server). See the Problems section.

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 installation.

Unpack the source code of oracle_fdw and change into the directory.
Then the software installation should be as simple as:

$ make
$ make install

For the second step you need write permission on the directories where PostgreSQL is installed.

If you want to build oracle_fdw in a source tree of PostgreSQL, use

$ make NO_PGXS=1

Building oracle_fdw on Windows:

To build oracle_fdw on Windows, you need:

• PostgreSQL headers and libraries. These can be found in the PostgreSQL installation directory.
• Oracle headers and libraries (SDK package for Instant Client).
• Microsoft Visual Studio 2013 or later.

To build, either open oracle_fdw\msvc\oracle_fdw.sln in the IDE, or:

• Open a development command prompt (either x86 or x64 depending on your PostgreSQL installation) and change to the oracle_fdw\msvc directory.

• Run (single command line):

  > msbuild oracle_fdw.sln /p:Configuration=(Debug or Release) ^
        /p:Platform=(Win32 or x64) ^
        /p:OracleClient=(path to Oracle Client/SDK) ^
        /p:PostgreSQL=(path to PostgreSQL installation)
  

  (The "^"s are line continuations; you can just put everything on a single line instead.)

When the build is complete, you will find oracle_fdw.dll in a subdirectory named for your build options, e.g. x64\Release.

If you use Visual Studio 2015 or later and you get errors about missing header files including sys/types.h, you must install the Universal CRT SDK (part of Visual Studio).

Installing the extension:

Make sure that the oracle_fdw shared library is installed in the PostgreSQL library directory and that oracle_fdw.control and the SQL files are in the PostgreSQL extension directory.

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.

To upgrade from an oracle_fdw version before 1.0.0, use

ALTER EXTENSION oracle_fdw UPDATE;

Note that the extension version as shown by the psql command \x or the system catalog pg_available_extensions is not the installed version of oracle_fdw. To get the oracle_fdw version, use the function oracle_diag.

Running the regression tests:

Unless you are developing oracle_fdw or want to test its functionality on an exotic platform, you don't have to do this.

For the regression tests to work, you must have a PostgreSQL cluster (9.3 or better) and an Oracle server (10.1 or better with Locator or Spatial) running, and the oracle_fdw binaries must be installed.
The regression tests will create a database called contrib_regression and run a number of tests. For the PostGIS regression tests to succeed, the PostGIS binaries must be installed.

The Oracle database must be prepared as follows:

• A user scott with password tiger must exist (unless you want to edit the regression test scripts). The user needs CREATE SESSION and CREATE TABLE system privileges and enough quota on its default tablespace, as well as SELECT privileges on V$SQL and V$SQL_PLAN.

• Two tables must be created as follows:

    CREATE TABLE scott.typetest1 (
       id  NUMBER(5)
          CONSTRAINT typetest1_pkey PRIMARY KEY,
       c   CHAR(10 CHAR),
       nc  NCHAR(10),
       vc  VARCHAR2(10 CHAR),
       nvc NVARCHAR2(10),
       lc  CLOB,
       r   RAW(10),
       u   RAW(16),
       lb  BLOB,
       lr  LONG RAW,
       b   NUMBER(1),
       num NUMBER(7,5),
       fl  BINARY_FLOAT,
       db  BINARY_DOUBLE,
       d   DATE,
       ts  TIMESTAMP WITH TIME ZONE,
       ids INTERVAL DAY TO SECOND,
       iym INTERVAL YEAR TO MONTH
    ) SEGMENT CREATION IMMEDIATE;
  
    CREATE TABLE scott.gis (
       id  NUMBER(5) PRIMARY KEY,
       g   MDSYS.SDO_GEOMETRY
    ) SEGMENT CREATION IMMEDIATE;
  

Set the environment for the PostgreSQL server so that it can establish an Oracle connection without connect string: If the Oracle server is on the same machine, set the environment variables ORACLE_SID and ORACLE_HOME appropriately, for a remote server set the environment variable TWO_TASK (or LOCAL on Windows) to the connect string.

The regression tests are run as follows:

$ make installcheck

7 Internals

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 row count can be configured with the prefetch table option and is set to 200 by default.

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.
Unfortunately the Oracle implementation of SERIALIZABLE has certain quirks; see the Problems section for more.

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 beyond what a foreign data wrapper can reasonably provide.
Prepared statements involving Oracle are not supported for the same reason.

8 Problems

Encoding

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 the Options section.

Limited functionality in old Oracle versions

• The definition of the Oracle system catalogs V$SQL and V$SQL_PLAN has changed with Oracle 10.1. Using EXPLAIN VERBOSE with older Oracle server versions will result in errors like:

  ERROR:  error describing query: OCIStmtExecute failed to execute
          remote query for sql_id
  DETAIL:  ORA-00904: "LAST_ACTIVE_TIME": invalid identifier
  

  There is no plan to fix this, since Oracle 9i has been out of Extended Support since 2010 and the functionality is not essential.

• IMPORT FOREIGN SCHEMA throws the following error with Oracle server 8i:

  ERROR:  error importing foreign schema: OCIStmtExecute failed to execute
          column query
  DETAIL:  ORA-00904: invalid column name
  

  This is because the view ALL_TAB_COLUMNS lacks the column CHAR_LENGTH, which was added in Oracle 9i.

LDAP libraries

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:

../../../libraries/libldap/getentry.c:29: ldap_first_entry:
 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.

Serialization errors

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.

A much nastier problem is that concurrent inserts can sometimes cause serialization errors when an index page is split concurrently with a modifying serializable transaction (see Oracle document 160593.1).

Oracle claims that this is not a bug, and the suggested solution is to retry the transaction that got a serialization error.

Oracle bugs

This is a list of Oracle bugs that have affected oracle_fdw in the past.

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 8.1.7.4 (install one-off patch 2728408) or Oracle server 9.2 (install Patch Set 9.2.0.4 or better).

Missing Oracle client DLL (on Windows only)

The following error message (from any query involving an Oracle foreign table) indicates that PostgreSQL cannot find the Oracle client library:

ERROR:  Oracle client library (oci.dll) not found
DETAIL:  No Oracle client is installed, or your system is configured incorrectly.
HINT:  Verify that the PATH variable includes the Oracle client.

Make sure that the path to oci.dll is in the PATH environment variable of the user running the PostgreSQL server. If it is running as a Windows service, this is the system environment, and you must restart the service after changing it.

If updating the environment does not work, you may be using a PostgreSQL distribution that provides its own environment variables, hiding the Oracle client. See the bug report at laurenz#160 for more information.

9 Support

If you want to report a problem with oracle_fdw, and the name of the foreign server is (for example) "ora_serv", please include the output of

SELECT oracle_diag('ora_serv');

in your problem report. If that causes an error, please also include the output of

SELECT oracle_diag();

If you have a problem or question or any kind of feedback, the preferred option is to open an issue on GitHub:
https://github.com/laurenz/oracle_fdw/issues
This requires a GitHub account.