This Foreign Data Wrapper error occurs when a column attribute number referenced in an FDW operation is invalid or out of range, typically after altering table structure without updating foreign table definitions.
The `fdw_invalid_column_number` error (error code HV008) is part of PostgreSQL's SQL/MED (SQL Management of External Data) error class. This error is raised when a Foreign Data Wrapper attempts to access or manipulate a column using an invalid attribute number. Foreign Data Wrappers use internal attribute numbers (column positions) to identify columns during operations. This error typically occurs when there's a mismatch between the column numbers the FDW expects and what actually exists in the foreign table definition. The most common cause is structural changes to the foreign table (like dropping or reordering columns) that aren't properly synchronized with the remote data source or cached metadata. Unlike regular tables, foreign tables act as proxies to external data sources. When you modify a foreign table's structure using DDL commands like `ALTER FOREIGN TABLE`, PostgreSQL doesn't automatically verify consistency with the remote server. This responsibility falls on the database administrator, making it easy to create mismatches that trigger HV008 errors.
First, check the structure of your foreign table and compare it with the actual remote table:
-- Check local foreign table structure
\d+ foreign_table_name
-- If using postgres_fdw, check remote table structure
SELECT column_name, data_type, ordinal_position
FROM information_schema.columns
WHERE table_name = 'remote_table_name'
ORDER BY ordinal_position;Look for discrepancies in:
- Number of columns (local vs remote)
- Column order (attribute numbers)
- Column names and data types
If you find mismatches, this confirms the source of the HV008 error.
The safest way to fix column mismatches is to recreate the foreign table:
-- Save any foreign table options first
\d+ foreign_table_name
-- Drop the foreign table (this does NOT affect the remote data)
DROP FOREIGN TABLE IF EXISTS foreign_table_name;
-- Recreate with correct column definitions matching remote table
CREATE FOREIGN TABLE foreign_table_name (
id integer,
name text,
created_at timestamp
) SERVER foreign_server
OPTIONS (schema_name 'public', table_name 'remote_table_name');Important: Ensure column order matches the remote table if your FDW uses positional mapping (like oracle_fdw). For postgres_fdw, column matching is by name, not position.
If you need to preserve foreign table permissions and dependencies, use ALTER FOREIGN TABLE:
-- Add missing columns
ALTER FOREIGN TABLE foreign_table_name
ADD COLUMN new_column_name data_type;
-- Drop extra columns (use CASCADE if other objects depend on them)
ALTER FOREIGN TABLE foreign_table_name
DROP COLUMN outdated_column CASCADE;
-- Rename columns to match remote table
ALTER FOREIGN TABLE foreign_table_name
RENAME COLUMN old_name TO new_name;Note: PostgreSQL does not automatically verify consistency with the remote server when you alter foreign tables. You must manually ensure the definitions match.
Some FDWs cache metadata about remote tables. Force a refresh by clearing the connection:
-- For postgres_fdw, close existing connections to the foreign server
-- This forces new connections to fetch fresh metadata
SELECT pg_catalog.pg_terminate_backend(pid)
FROM pg_stat_activity
WHERE datname = current_database()
AND application_name LIKE 'postgres_fdw%';
-- Or restart the PostgreSQL service (more disruptive)
-- sudo systemctl restart postgresqlThen try your query again. The FDW will establish fresh connections with updated metadata.
Verify the foreign table is working correctly:
-- Test basic SELECT
SELECT * FROM foreign_table_name LIMIT 5;
-- Test column count matches expectation
SELECT COUNT(*) as column_count
FROM information_schema.columns
WHERE table_name = 'foreign_table_name';
-- Test specific column access
SELECT column_name FROM foreign_table_name LIMIT 1;If these queries succeed without HV008 errors, your foreign table is properly synchronized.
For Custom FDW Developers: If you're developing a custom FDW and encountering HV008 errors, review your callback implementations:
- In IterateForeignScan, BeginForeignScan, or GetForeignRelSize callbacks, ensure you're not accessing column attribute numbers beyond RelationGetNumberOfAttributes(rel)
- Use the FDW helper function GetForeignColumnOptions(Oid relid, AttrNumber attnum) to safely retrieve column metadata
- Validate that target attribute numbers in your execution state match the actual foreign table definition
- Remember that dropped columns leave "holes" in attribute numbering - use TupleDescAttr() to safely access column descriptors
- When building tuple slots, ensure you're populating exactly the number of attributes defined in the tuple descriptor
Positional vs Name-Based Matching: Different FDWs handle column mapping differently:
- postgres_fdw: Matches columns by name, allowing different column orders between local and remote tables
- oracle_fdw: Matches columns by position, requiring identical column order
- Custom FDWs may implement either approach
Check your specific FDW documentation to understand its column mapping behavior.
Remote Table Changes: If the remote table structure changes frequently, consider:
- Implementing monitoring to detect schema drift between local foreign table definitions and remote tables
- Using views on the remote side to provide stable interfaces
- Documenting foreign table creation dates and last verified sync times
- Automating foreign table recreation as part of deployment pipelines when remote schemas change
Transaction Safety: DROP FOREIGN TABLE and ALTER FOREIGN TABLE are DDL operations that can't be rolled back once committed. Always test in a non-production environment first, especially if the foreign table has dependent views or functions.
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