oracle_fdw: Foreign Data Wrapper for Oracle

December 18, 2023

Summary: 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 article contains the following sections:

  1. Cookbook
  2. Objects created by the extension
  3. Options
  4. Usage

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 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 installed.

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');

You can use other naming methods or local connections, see the description of the option dbserver below.

It is a good idea to use a superuser only where really necessary, so let’s allow a normal user to use the foreign server (this is not required for the example to work, but it is recommended):

pgdb=# GRANT USAGE ON FOREIGN SERVER oradb TO pguser;

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.

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.

FUNCTION oracle_execute(server name, stmt text) RETURNS void

This function can be used to execute arbitrary SQL statements on the remote Oracle server. That will only work with statements that do not return results (typically DDL statements).

Be careful when using this function, since it might disturb the transaction management of oracle_fdw. Remember that running a DDL statement in Oracle will issue an implicit COMMIT. You are best advised to use this function outside of multi-statement transactions.

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.

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.

  • isolation_level (optional, defaults to serializable)

    The transaction isolation level to use at the Oracle database. The value can be serializable, read_committed or read_only.

    Note that the Oracle table can be queried more than once during a single PostgreSQL statement (for example, during a nested loop join). To make sure that no inconsistencies caused by race conditions with concurrent transactions can occur, the transaction isolation level must guarantee read stability. This is only guaranteed with Oracle’s SERIALIZABLE or READ ONLY isolation levels.

    Unfortunately Oracle’s implementation of SERIALIZABLE is rather bad and causes serialization errors (ORA-08177) in unexpected situations, like inserts into the table. Using READ COMMITTED transactions works around this problem, but there is a risk of inconsistencies. If you want to use it, check your execution plans whether the foreign scan could be executed more than once.

  • nchar (boolean, optional, defaults to off)

    Setting this option to on chooses a more expensive character conversion on the Oracle side. This is required if you are using a single-byte Oracle database character set, but have NCHAR or NVARCHAR2 columns containing characters that cannot be represented in the database character set.

    Setting nchar to on has a noticable performance impact, and it causes ORA-01461 errors with UPDATE statements that set strings over 2000 bytes (or 16383 if you have MAX_STRING_SIZE = EXTENDED). This error seems to be an Oracle bug.

  • set_timezone (boolean, optional, defaults to off)

    Setting this option to on sets the Oracle session time zone to the current value of the PostgreSQL parameter timezone when the connection to Oracle is made. This is only useful if you plan to use Oracle columns of type TIMESTAMP WITH LOCAL TIME ZONE and want to translate them to timestamp without time zone in PostgreSQL.

    Note that if you change timezone after the Oracle connection has been established, oracle_fdw won’t change the Oracle session time zone. You can call oracle_close_connections() in that case, so that a new connection is opened the next time you access a foreign table.

    If Oracle does not recognize the time zone, connections will fail with an error like

    ORA-01882: timezone region not found
    

    In that case, either use a different timezone or leave the option set off and set the environment variable ORA_SDTZ to an appropriate value in the environment of the PostgreSQL server.

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.

  • dblink (optional)

    The Oracle database link through which the table is accessed. 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 only.

  • max_long (optional, defaults to “32767”)

    The maximal length of any LONG, LONG RAW and XMLTYPE 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.

  • 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 “50”)

    Sets the number of rows that will be fetched with a single round-trip between PostgreSQL and Oracle during a foreign table scan. The value must be between 1 and 1000.

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

Note that there is no prefetching if the Oracle table contains columns of the type MDSYS.SDO_GEOMETRY.

  • lob_prefetch (optional, defaults to “1048576”)

    Sets the number of bytes that are prefetched for BLOB, CLOB and BFILE values. LOBs that exceed that size will require additional round trips between PostgreSQL and Oracle, so setting this value bigger than the size of your typical LOB will be good for performance. Choosing bigger values for this option can allocate more memory on the server side, but will boost performance for large LOBs.

Column options

  • 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.

  • strip_zeros (optional, defaults to “false”)

    If set to yes/on/true, ASCII 0 characters will be removed from the string during transfer. Such characters are valid in Oracle but not in PostgreSQL, so they will cause an error when read by oracle_fdw. This option only make sense for character, character varying and text columns.

Usage

Oracle permissions

The Oracle user will obviously need CREATE SESSION privilege and the right to select from the table or view in question. Note that oracle_fdw accesses the Oracle table at query planning time to get its definition. This happens before permissions on the foreign table are checked. Consequently, you may receive an Oracle error if you try to access a foreign table on which you have no permissions in PostgreSQL. This is expected and no security problem.

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
XMLTYPE                  | xml, 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.

Inserting or updating XMLTYPE only works with values that do not exceed the maximum length of the VARCHAR2 data type (4000 or 32767, depending on the MAX_STRING_SIZE parameter).

NCLOB is currently not supported because Oracle cannot automatically convert it to the client encoding.

If you want to convert TIMESTAMP WITH LOCAL TIME ZONE to timestamp, consider setting the set_timezone option on the foreign server.

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 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.
  • 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

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.

Since oracle_fdw uses serialized transactions by default, 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.

It is possible to use a different transaction isolation level, see Foreign server options for a discussion.

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).

ANALYZE

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.

  • These are the 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).
  • collation: the collation used for case folding for the lower and smart options of case

    The default value is default which is the database’s default collation. Only collations in the pg_catalog schema are supported. See the collname values in the pg_collation catalog for a list of possible values.

    • dblink: the Oracle database link through which the schema is accessed

    This name must be written exactly as it occurs in Oracle’s system catalog, so normally consist of uppercase letters only.

    • readonly: sets the readonly option on all imported tables

    See the Options section for details.

  • max_long: sets the max_long option on all imported tables

    See the Options section for details.

  • sample_percent: sets the sample_percent option on all imported tables

    See the Options section for details.

    • prefetch: sets the prefetch option on all imported tables

      See the Options section for details.

    • lob_prefetch: sets the lob_prefetch option on all imported tables

      See the Options section for details.

    • nchar: sets the nchar option on all imported tables

      See the Options section for details.

    • set_timezone: sets the set_timezone option on all imported tables

      See the Options section for details.

  • 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.

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