postgres_fdw module provides the foreign-data wrapper
postgres_fdw, which can be used to access data
stored in external PostgreSQL servers.
The functionality provided by this module overlaps substantially
with the functionality of the older dblink module.
postgres_fdw provides more transparent and
standards-compliant syntax for accessing remote tables, and can give
better performance in many cases.
To prepare for remote access using
postgres_fdwextension using CREATE EXTENSION.
Create a foreign server object, using CREATE SERVER, to represent each remote database you want to connect to. Specify connection information, except
password, as options of the server object.
Create a user mapping, using CREATE USER MAPPING, for each database user you want to allow to access each foreign server. Specify the remote user name and password to use as
passwordoptions of the user mapping.
Create a foreign table, using CREATE FOREIGN TABLE or IMPORT FOREIGN SCHEMA, for each remote table you want to access. The columns of the foreign table must match the referenced remote table. You can, however, use table and/or column names different from the remote table's, if you specify the correct remote names as options of the foreign table object.
Now you need only
SELECT from a foreign table to access
the data stored in its underlying remote table. You can also modify
the remote table using
DELETE. (Of course, the remote user you have specified
in your user mapping must have privileges to do these things.)
postgres_fdw currently lacks support for
INSERT statements with an
ON CONFLICT DO
UPDATE clause. However, the
ON CONFLICT DO NOTHING
clause is supported, provided a unique index inference specification
It is generally recommended that the columns of a foreign table be declared
with exactly the same data types, and collations if applicable, as the
referenced columns of the remote table. Although
is currently rather forgiving about performing data type conversions at
need, surprising semantic anomalies may arise when types or collations do
not match, due to the remote server interpreting
slightly differently from the local server.
Note that a foreign table can be declared with fewer columns, or with a different column order, than its underlying remote table has. Matching of columns to the remote table is by name, not position.
F.34.1. FDW Options of postgres_fdw
F.34.1.1. Connection Options
A foreign server using the
postgres_fdw foreign data wrapper
can have the same options that libpq accepts in
connection strings, as described in Section 33.1.2,
except that these options are not allowed:
password(specify these in a user mapping, instead)
client_encoding(this is automatically set from the local server encoding)
fallback_application_name(always set to
Only superusers may connect to foreign servers without password
authentication, so always specify the
for user mappings belonging to non-superusers.
F.34.1.2. Object Name Options
These options can be used to control the names used in SQL statements sent to the remote PostgreSQL server. These options are needed when a foreign table is created with names different from the underlying remote table's names.
This option, which can be specified for a foreign table, gives the schema name to use for the foreign table on the remote server. If this option is omitted, the name of the foreign table's schema is used.
This option, which can be specified for a foreign table, gives the table name to use for the foreign table on the remote server. If this option is omitted, the foreign table's name is used.
This option, which can be specified for a column of a foreign table, gives the column name to use for the column on the remote server. If this option is omitted, the column's name is used.
F.34.1.3. Cost Estimation Options
postgres_fdw retrieves remote data by executing queries
against remote servers, so ideally the estimated cost of scanning a
foreign table should be whatever it costs to be done on the remote
server, plus some overhead for communication. The most reliable way to
get such an estimate is to ask the remote server and then add something
for overhead — but for simple queries, it may not be worth the cost
of an additional remote query to get a cost estimate.
postgres_fdw provides the following options to control
how cost estimation is done:
This option, which can be specified for a foreign table or a foreign server, controls whether
EXPLAINcommands to obtain cost estimates. A setting for a foreign table overrides any setting for its server, but only for that table. The default is
This option, which can be specified for a foreign server, is a numeric value that is added to the estimated startup cost of any foreign-table scan on that server. This represents the additional overhead of establishing a connection, parsing and planning the query on the remote side, etc. The default value is
This option, which can be specified for a foreign server, is a numeric value that is used as extra cost per-tuple for foreign-table scans on that server. This represents the additional overhead of data transfer between servers. You might increase or decrease this number to reflect higher or lower network delay to the remote server. The default value is
use_remote_estimate is true,
postgres_fdw obtains row count and cost estimates from the
remote server and then adds
fdw_tuple_cost to the cost estimates. When
use_remote_estimate is false,
postgres_fdw performs local row count and cost estimation
and then adds
fdw_tuple_cost to the cost estimates. This local
estimation is unlikely to be very accurate unless local copies of the
remote table's statistics are available. Running
ANALYZE on the foreign table is the way to update
the local statistics; this will perform a scan of the remote table and
then calculate and store statistics just as though the table were local.
Keeping local statistics can be a useful way to reduce per-query planning
overhead for a remote table — but if the remote table is
frequently updated, the local statistics will soon be obsolete.
F.34.1.4. Remote Execution Options
By default, only
WHERE clauses using built-in operators and
functions will be considered for execution on the remote server. Clauses
involving non-built-in functions are checked locally after rows are
fetched. If such functions are available on the remote server and can be
relied on to produce the same results as they do locally, performance can
be improved by sending such
WHERE clauses for remote
execution. This behavior can be controlled using the following option:
This option is a comma-separated list of names of PostgreSQL extensions that are installed, in compatible versions, on both the local and remote servers. Functions and operators that are immutable and belong to a listed extension will be considered shippable to the remote server. This option can only be specified for foreign servers, not per-table.
When using the
extensionsoption, it is the user's responsibility that the listed extensions exist and behave identically on both the local and remote servers. Otherwise, remote queries may fail or behave unexpectedly.
This option specifies the number of rows
postgres_fdwshould get in each fetch operation. It can be specified for a foreign table or a foreign server. The option specified on a table overrides an option specified for the server. The default is
F.34.1.5. Updatability Options
By default all foreign tables using
postgres_fdw are assumed
to be updatable. This may be overridden using the following option:
This option controls whether
postgres_fdwallows foreign tables to be modified using
DELETEcommands. It can be specified for a foreign table or a foreign server. A table-level option overrides a server-level option. The default is
Of course, if the remote table is not in fact updatable, an error would occur anyway. Use of this option primarily allows the error to be thrown locally without querying the remote server. Note however that the
information_schemaviews will report a
postgres_fdwforeign table to be updatable (or not) according to the setting of this option, without any check of the remote server.
F.34.1.6. Importing Options
postgres_fdw is able to import foreign table definitions
using IMPORT FOREIGN SCHEMA. This command creates
foreign table definitions on the local server that match tables or
views present on the remote server. If the remote tables to be imported
have columns of user-defined data types, the local server must have
compatible types of the same names.
Importing behavior can be customized with the following options
(given in the
IMPORT FOREIGN SCHEMA command):
This option controls whether column
COLLATEoptions are included in the definitions of foreign tables imported from a foreign server. The default is
true. You might need to turn this off if the remote server has a different set of collation names than the local server does, which is likely to be the case if it's running on a different operating system.
This option controls whether column
DEFAULTexpressions are included in the definitions of foreign tables imported from a foreign server. The default is
false. If you enable this option, be wary of defaults that might get computed differently on the local server than they would be on the remote server;
nextval()is a common source of problems. The
IMPORTwill fail altogether if an imported default expression uses a function or operator that does not exist locally.
This option controls whether column
NOT NULLconstraints are included in the definitions of foreign tables imported from a foreign server. The default is
Note that constraints other than
NOT NULL will never be
imported from the remote tables. Although PostgreSQL
CHECK constraints on foreign tables, there is no
provision for importing them automatically, because of the risk that a
constraint expression could evaluate differently on the local and remote
servers. Any such inconsistency in the behavior of a
constraint could lead to hard-to-detect errors in query optimization.
So if you wish to import
CHECK constraints, you must do so
manually, and you should verify the semantics of each one carefully.
For more detail about the treatment of
CHECK constraints on
foreign tables, see CREATE FOREIGN TABLE.
Tables or foreign tables which are partitions of some other table are automatically excluded. Partitioned tables are imported, unless they are a partition of some other table. Since all data can be accessed through the partitioned table which is the root of the partitioning hierarchy, this approach should allow access to all the data without creating extra objects.
F.34.2. Connection Management
postgres_fdw establishes a connection to a
foreign server during the first query that uses a foreign table
associated with the foreign server. This connection is kept and
re-used for subsequent queries in the same session. However, if
multiple user identities (user mappings) are used to access the foreign
server, a connection is established for each user mapping.
F.34.3. Transaction Management
During a query that references any remote tables on a foreign server,
postgres_fdw opens a transaction on the
remote server if one is not already open corresponding to the current
local transaction. The remote transaction is committed or aborted when
the local transaction commits or aborts. Savepoints are similarly
managed by creating corresponding remote savepoints.
The remote transaction uses
isolation level when the local transaction has
isolation level; otherwise it uses
isolation level. This choice ensures that if a query performs multiple
table scans on the remote server, it will get snapshot-consistent results
for all the scans. A consequence is that successive queries within a
single transaction will see the same data from the remote server, even if
concurrent updates are occurring on the remote server due to other
activities. That behavior would be expected anyway if the local
isolation level, but it might be surprising for a
COMMITTED local transaction. A future
PostgreSQL release might modify these rules.
Note that it is currently not supported by
postgres_fdw to prepare the remote transaction for
F.34.4. Remote Query Optimization
postgres_fdw attempts to optimize remote queries to reduce
the amount of data transferred from foreign servers. This is done by
WHERE clauses to the remote server for
execution, and by not retrieving table columns that are not needed for
the current query. To reduce the risk of misexecution of queries,
WHERE clauses are not sent to the remote server unless they use
only data types, operators, and functions that are built-in or belong to an
extension that's listed in the foreign server's
option. Operators and functions in such clauses must
IMMUTABLE as well.
postgres_fdw attempts to optimize the query execution by
sending the whole query to the remote server if there are no query
WHERE clauses that cannot be sent to the remote server,
no local joins for the query, no row-level local
AFTER triggers on the target table, and no
CHECK OPTION constraints from parent views.
expressions to assign to target columns must use only built-in data types,
IMMUTABLE operators, or
to reduce the risk of misexecution of the query.
postgres_fdw encounters a join between foreign tables on
the same foreign server, it sends the entire join to the foreign server,
unless for some reason it believes that it will be more efficient to fetch
rows from each table individually, or unless the table references involved
are subject to different user mappings. While sending the
clauses, it takes the same precautions as mentioned above for the
The query that is actually sent to the remote server for execution can
be examined using
F.34.5. Remote Query Execution Environment
In the remote sessions opened by
the search_path parameter is set to
pg_catalog, so that only built-in objects are visible
without schema qualification. This is not an issue for queries
postgres_fdw itself, because it always
supplies such qualification. However, this can pose a hazard for
functions that are executed on the remote server via triggers or rules
on remote tables. For example, if a remote table is actually a view,
any functions used in that view will be executed with the restricted
search path. It is recommended to schema-qualify all names in such
functions, or else attach
SET search_path options
(see CREATE FUNCTION) to such functions
to establish their expected search path environment.
postgres_fdw likewise establishes remote session settings
for various parameters:
These are less likely to be problematic than
can be handled with function
SET options if the need arises.
It is not recommended that you override this behavior by
changing the session-level settings of these parameters; that is likely
postgres_fdw to malfunction.
F.34.6. Cross-Version Compatibility
postgres_fdw can be used with remote servers dating back
to PostgreSQL 8.3. Read-only capability is available
back to 8.1. A limitation however is that
generally assumes that immutable built-in functions and operators are
safe to send to the remote server for execution, if they appear in a
WHERE clause for a foreign table. Thus, a built-in
function that was added since the remote server's release might be sent
to it for execution, resulting in “function does not exist” or
a similar error. This type of failure can be worked around by
rewriting the query, for example by embedding the foreign table
reference in a sub-
OFFSET 0 as an
optimization fence, and placing the problematic function or operator
outside the sub-
Here is an example of creating a foreign table with
postgres_fdw. First install the extension:
CREATE EXTENSION postgres_fdw;
Then create a foreign server using CREATE SERVER.
In this example we wish to connect to a PostgreSQL server
126.96.36.199 listening on
5432. The database to which the connection is made
foreign_db on the remote server:
CREATE SERVER foreign_server FOREIGN DATA WRAPPER postgres_fdw OPTIONS (host '188.8.131.52', port '5432', dbname 'foreign_db');
A user mapping, defined with CREATE USER MAPPING, is needed as well to identify the role that will be used on the remote server:
CREATE USER MAPPING FOR local_user SERVER foreign_server OPTIONS (user 'foreign_user', password 'password');
Now it is possible to create a foreign table with
CREATE FOREIGN TABLE. In this example we
wish to access the table named
on the remote server. The local name for it will
CREATE FOREIGN TABLE foreign_table ( id integer NOT NULL, data text ) SERVER foreign_server OPTIONS (schema_name 'some_schema', table_name 'some_table');
It's essential that the data types and other properties of the columns
CREATE FOREIGN TABLE match the actual remote table.
Column names must match as well, unless you attach
options to the individual columns to show how they are named in the remote
In many cases, use of IMPORT FOREIGN SCHEMA is
preferable to constructing foreign table definitions manually.