The 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.
But 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_fdw:
Install the postgres_fdw extension 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 user and
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
user and password options 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 INSERT, UPDATE,
DELETE, COPY, or
TRUNCATE.
(Of course, the remote user you have specified in your user mapping must
have privileges to do these things.)
Note that the ONLY option specified in
SELECT, UPDATE,
DELETE or TRUNCATE
has no effect when accessing or modifying the remote table.
Note that 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
is omitted.
Note also that postgres_fdw supports row movement
invoked by UPDATE statements executed on partitioned
tables, but it currently does not handle the case where a remote partition
chosen to insert a moved row into is also an UPDATE
target partition that will be updated elsewhere in the same command.
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 postgres_fdw
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 query conditions
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.
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 34.1.2,
except that these options are not allowed or have special handling:
user, password and sslpassword (specify these
in a user mapping, instead, or use a service file)
client_encoding (this is automatically set from the local
server encoding)
fallback_application_name (always set to
postgres_fdw)
sslkey and sslcert - these may
appear in either or both a connection and a user
mapping. If both are present, the user mapping setting overrides the
connection setting.
Only superusers may create or modify user mappings with the
sslcert or sslkey settings.
Only superusers may connect to foreign servers without password
authentication, so always specify the password option
for user mappings belonging to non-superusers.
A superuser may override this check on a per-user-mapping basis by setting
the user mapping option password_required 'false', e.g.,
ALTER USER MAPPING FOR some_non_superuser SERVER loopback_nopw OPTIONS (ADD password_required 'false');
To prevent unprivileged users from exploiting the authentication rights of the unix user the postgres server is running as to escalate to superuser rights, only the superuser may set this option on a user mapping.
Care is required to ensure that this does not allow the mapped
user the ability to connect as superuser to the mapped database per
CVE-2007-3278 and CVE-2007-6601. Don't set
password_required=false
on the public role. Keep in mind that the mapped
user can potentially use any client certificates,
.pgpass,
.pg_service.conf etc in the unix home directory of the
system user the postgres server runs as. They can also use any trust
relationship granted by authentication modes like peer
or ident authentication.
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.
schema_nameThis 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.
table_nameThis 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.
column_nameThis 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.
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.
So postgres_fdw provides the following options to control
how cost estimation is done:
use_remote_estimate
This option, which can be specified for a foreign table or a foreign
server, controls whether postgres_fdw issues remote
EXPLAIN commands to obtain cost estimates.
A setting for a foreign table overrides any setting for its server,
but only for that table.
The default is false.
fdw_startup_cost
This option, which can be specified for a foreign server, is a floating
point 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 100.
fdw_tuple_cost
This option, which can be specified for a foreign server, is a floating
point 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 0.01.
When use_remote_estimate is true,
postgres_fdw obtains row count and cost estimates from the
remote server and then adds fdw_startup_cost and
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_startup_cost and
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.
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:
extensionsThis 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 extensions option, 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.
fetch_size
This option specifies the number of rows postgres_fdw
should 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 100.
batch_size
This option specifies the number of rows postgres_fdw
should insert in each insert 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 1.
Note the actual number of rows postgres_fdw inserts at
once depends on the number of columns and the provided
batch_size value. The batch is executed as a single
query, and the libpq protocol (which postgres_fdw
uses to connect to a remote server) limits the number of parameters in a
single query to 65535. When the number of columns * batch_size
exceeds the limit, the batch_size will be adjusted to
avoid an error.
postgres_fdw supports asynchronous execution, which
runs multiple parts of an Append node
concurrently rather than serially to improve performance.
This execution can be controlled using the following option:
async_capable
This option controls whether postgres_fdw allows
foreign tables to be scanned concurrently for asynchronous execution.
It can be specified for a foreign table or a foreign server.
A table-level option overrides a server-level option.
The default is false.
In order to ensure that the data being returned from a foreign server
is consistent, postgres_fdw will only open one
connection for a given foreign server and will run all queries against
that server sequentially even if there are multiple foreign tables
involved, unless those tables are subject to different user mappings.
In such a case, it may be more performant to disable this option to
eliminate the overhead associated with running queries asynchronously.
Asynchronous execution is applied even when an
Append node contains subplan(s) executed
synchronously as well as subplan(s) executed asynchronously.
In such a case, if the asynchronous subplans are ones processed using
postgres_fdw, tuples from the asynchronous
subplans are not returned until after at least one synchronous subplan
returns all tuples, as that subplan is executed while the asynchronous
subplans are waiting for the results of asynchronous queries sent to
foreign servers.
This behavior might change in a future release.
By default all foreign tables using postgres_fdw are assumed
to be updatable. This may be overridden using the following option:
updatable
This option controls whether postgres_fdw allows foreign
tables to be modified using INSERT, UPDATE and
DELETE commands. It can be specified for a foreign table
or a foreign server. A table-level option overrides a server-level
option.
The default is true.
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_schema views will report a
postgres_fdw foreign table to be updatable (or not)
according to the setting of this option, without any check of the
remote server.
By default all foreign tables using postgres_fdw are assumed
to be truncatable. This may be overridden using the following option:
truncatable
This option controls whether postgres_fdw allows
foreign tables to be truncated using the TRUNCATE
command. It can be specified for a foreign table or a foreign server.
A table-level option overrides a server-level option.
The default is true.
Of course, if the remote table is not in fact truncatable, an error would occur anyway. Use of this option primarily allows the error to be thrown locally without querying the remote server.
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):
import_collate
This option controls whether column COLLATE options
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.
If you do so, however, there is a very severe risk that the imported
table columns' collations will not match the underlying data, resulting
in anomalous query behavior.
Even when this parameter is set to true, importing
columns whose collation is the remote server's default can be risky.
They will be imported with COLLATE "default", which
will select the local server's default collation, which could be
different.
import_default
This option controls whether column DEFAULT expressions
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 IMPORT will fail altogether if an imported default
expression uses a function or operator that does not exist locally.
import_generated
This option controls whether column GENERATED expressions
are included in the definitions of foreign tables imported
from a foreign server. The default is true.
The IMPORT will fail altogether if an imported generated
expression uses a function or operator that does not exist locally.
import_not_null
This option controls whether column NOT NULL
constraints are included in the definitions of foreign tables imported
from a foreign server. The default is true.
Note that constraints other than NOT NULL will never be
imported from the remote tables. Although PostgreSQL
does support 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 check
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
imported only when they are explicitly specified in
LIMIT TO clause. Otherwise they are automatically
excluded from IMPORT FOREIGN SCHEMA.
Since all data can be accessed through the partitioned table
which is the root of the partitioning hierarchy, importing only
partitioned tables should allow access to all the data without
creating extra objects.
By default, all connections that postgres_fdw
establishes to foreign servers are kept open in the local session
for re-use.
keep_connections
This option controls whether postgres_fdw keeps
the connections to the foreign server open so that subsequent
queries can re-use them. It can only be specified for a foreign server.
The default is on. If set to off,
all connections to this foreign server will be discarded at the end of
each transaction.
postgres_fdw_get_connections(OUT server_name text, OUT valid boolean) returns setof record
This function returns the foreign server names of all the open
connections that postgres_fdw established from
the local session to the foreign servers. It also returns whether
each connection is valid or not. false is returned
if the foreign server connection is used in the current local
transaction but its foreign server or user mapping is changed or
dropped (Note that server name of an invalid connection will be
NULL if the server is dropped),
and then such invalid connection will be closed at
the end of that transaction. true is returned
otherwise. If there are no open connections, no record is returned.
Example usage of the function:
postgres=# SELECT * FROM postgres_fdw_get_connections() ORDER BY 1; server_name | valid -------------+------- loopback1 | t loopback2 | f
postgres_fdw_disconnect(server_name text) returns boolean
This function discards the open connections that are established by
postgres_fdw from the local session to
the foreign server with the given name. Note that there can be
multiple connections to the given server using different user mappings.
If the connections are used in the current local transaction,
they are not disconnected and warning messages are reported.
This function returns true if it disconnects
at least one connection, otherwise false.
If no foreign server with the given name is found, an error is reported.
Example usage of the function:
postgres=# SELECT postgres_fdw_disconnect('loopback1');
postgres_fdw_disconnect
-------------------------
t
postgres_fdw_disconnect_all() returns boolean
This function discards all the open connections that are established by
postgres_fdw from the local session to
foreign servers. If the connections are used in the current local
transaction, they are not disconnected and warning messages are reported.
This function returns true if it disconnects
at least one connection, otherwise false.
Example usage of the function:
postgres=# SELECT postgres_fdw_disconnect_all(); postgres_fdw_disconnect_all ----------------------------- t
postgres_fdw establishes a connection to a
foreign server during the first query that uses a foreign table
associated with the foreign server. By default this connection
is kept and re-used for subsequent queries in the same session.
This behavior can be controlled using
keep_connections option for a foreign server. If
multiple user identities (user mappings) are used to access the foreign
server, a connection is established for each user mapping.
When changing the definition of or removing a foreign server or a user mapping, the associated connections are closed. But note that if any connections are in use in the current local transaction, they are kept until the end of the transaction. Closed connections will be re-established when they are necessary by future queries using a foreign table.
Once a connection to a foreign server has been established,
it's by default kept until the local or corresponding remote
session exits. To disconnect a connection explicitly,
keep_connections option for a foreign server
may be disabled, or
postgres_fdw_disconnect and
postgres_fdw_disconnect_all functions
may be used. For example, these are useful to close
connections that are no longer necessary, thereby releasing
connections on the foreign server.
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 SERIALIZABLE
isolation level when the local transaction has SERIALIZABLE
isolation level; otherwise it uses REPEATABLE READ
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
transaction uses SERIALIZABLE or REPEATABLE READ
isolation level, but it might be surprising for a READ
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
two-phase commit.
postgres_fdw attempts to optimize remote queries to reduce
the amount of data transferred from foreign servers. This is done by
sending query 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 extensions
option. Operators and functions in such clauses must
be IMMUTABLE as well.
For an UPDATE or DELETE query,
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 BEFORE or
AFTER triggers or stored generated columns on the target
table, and no CHECK OPTION constraints from parent
views. In UPDATE,
expressions to assign to target columns must use only built-in data types,
IMMUTABLE operators, or IMMUTABLE functions,
to reduce the risk of misexecution of the query.
When 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 JOIN
clauses, it takes the same precautions as mentioned above for the
WHERE clauses.
The query that is actually sent to the remote server for execution can
be examined using EXPLAIN VERBOSE.
In the remote sessions opened by postgres_fdw,
the search_path parameter is set to
just pg_catalog, so that only built-in objects are visible
without schema qualification. This is not an issue for queries
generated by 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:
TimeZone is set to UTC
DateStyle is set to ISO
IntervalStyle is set to postgres
extra_float_digits is set to 3 for remote
servers 9.0 and newer and is set to 2 for older versions
These are less likely to be problematic than search_path, but
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
to cause postgres_fdw to malfunction.
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 postgres_fdw
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-SELECT with OFFSET 0 as an
optimization fence, and placing the problematic function or operator
outside the sub-SELECT.
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
on host 192.83.123.89 listening on
port 5432. The database to which the connection is made
is named foreign_db on the remote server:
CREATE SERVER foreign_server
FOREIGN DATA WRAPPER postgres_fdw
OPTIONS (host '192.83.123.89', 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 some_schema.some_table
on the remote server. The local name for it will
be foreign_table:
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
declared in CREATE FOREIGN TABLE match the actual remote table.
Column names must match as well, unless you attach column_name
options to the individual columns to show how they are named in the remote
table.
In many cases, use of IMPORT FOREIGN SCHEMA is
preferable to constructing foreign table definitions manually.
Shigeru Hanada <shigeru.hanada@gmail.com>