Foreign data wrapper (FDW) errors occur when connecting to remote data sources. Common causes include missing extensions, misconfigured servers, invalid credentials, or missing handler functions. Fix by installing postgres_fdw, verifying server configuration, and ensuring proper user mappings.
A foreign data wrapper (FDW) error indicates a problem accessing external data sources from PostgreSQL. This occurs when PostgreSQL cannot communicate with a remote database or data source through an FDW extension. Errors can stem from missing extensions like postgres_fdw, improperly configured remote servers, incorrect authentication credentials, or undefined handler functions. The error typically appears when executing queries against foreign tables or during FDW setup.
Connect to your database as a superuser and create the postgres_fdw extension:
CREATE EXTENSION IF NOT EXISTS postgres_fdw;This installs the handler and validator functions needed for remote PostgreSQL access.
Define a foreign server pointing to your remote database:
CREATE SERVER remote_db
FOREIGN DATA WRAPPER postgres_fdw
OPTIONS (host '192.168.1.100', port '5432', dbname 'remote_database');Always explicitly reference the handler. If creating a custom FDW, use:
CREATE FOREIGN DATA WRAPPER custom_fdw
HANDLER public.custom_fdw_handler
VALIDATOR public.custom_fdw_validator;For each local user that needs access, create a user mapping:
CREATE USER MAPPING FOR current_user
SERVER remote_db
OPTIONS (user 'remote_user', password 'remote_password');Non-superusers MUST have a password specified in the user mapping. Do not omit this for security.
Option A - Import schema automatically (postgres_fdw only):
IMPORT FOREIGN SCHEMA public
FROM SERVER remote_db
INTO local_schema;Option B - Create foreign tables manually:
CREATE FOREIGN TABLE remote_table (
id integer,
name text,
created_at timestamp
) SERVER remote_db
OPTIONS (schema_name 'public', table_name 'table_name');Check that the FDW and server were created correctly:
SELECT * FROM information_schema.foreign_data_wrappers;
SELECT * FROM information_schema.foreign_servers;
SELECT * FROM information_schema.user_mappings;Test with a simple query to ensure connectivity:
SELECT * FROM remote_table LIMIT 1;If you see "could not connect to server" or "password authentication failed", review your server OPTIONS and user mapping credentials.
For Oracle FDW, ensure the Oracle client library (oci.dll or libclntsh.so) is installed and the PATH environment variable includes the Oracle client directory. For postgres_fdw performance, use use_remote_estimate 'true' to retrieve cost estimates from the remote server, and fetch_size '10000' to batch rows efficiently. When experiencing "function does not exist" errors on remote functions, wrap the foreign table in a sub-SELECT with OFFSET 0 to prevent pushdown of newer functions to older remote servers. Type mismatches between local and remote schemas can cause silent data corruption; always verify column types match exactly.
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