fdw_invalid_descriptor_field_identifier Error in PostgreSQL Error - HV091

What is fdw_invalid_descriptor_field_identifier and how do I fix it?

PostgreSQL raises SQLSTATE HV091 with condition name fdw_invalid_descriptor_field_identifier when a foreign data wrapper (FDW) calls a tuple descriptor field index that does not exist. The invalid reference prevents the FDW from mapping remote columns to local rows, so the query aborts.

The error surfaces after schema changes, mismatched FDW versions, or corrupted descriptors.

Fixing the underlying index mismatch restores normal query execution.

What Causes This Error?

Descriptor and column counts can drift when you ALTER FOREIGN TABLE without updating its OPTIONS or when the remote server changes its schema. The FDW still stores the old field indexes and fails at runtime.

Upgrading PostgreSQL or the FDW extension can change internal descriptor structures.

Calls compiled against the old layout hit invalid indexes and trigger HV091 immediately after the upgrade.

Manual C-language FDW code that computes field numbers incorrectly will also trip this safeguard, usually during development or custom patching.

How to Fix fdw_invalid_descriptor_field_identifier

First confirm column mismatches by comparing pg_attribute on the foreign table with the remote table definition.

If counts differ, drop and recreate the FOREIGN TABLE so PostgreSQL rebuilds the descriptor correctly.

When the FDW extension was just upgraded, run ALTER EXTENSION ... UPDATE or CREATE EXTENSION ... REPLACE to ensure all objects pick up the new binary. Restart PostgreSQL to flush any stale shared libraries.

For custom FDWs, rebuild the C module, verify GetForeignColumnInfo logic, and retest with regression cases.

Correcting the index math eliminates the error.

Common Scenarios and Solutions

Scenario: Added a new column on the remote database. Solution: Refresh the local foreign table definition with IMPORT FOREIGN SCHEMA or DROP/CREATE FOREIGN TABLE.

Scenario: Switched from postgres_fdw 1.1 to 1.2. Solution: ALTER EXTENSION postgres_fdw UPDATE; followed by a server restart.

Scenario: Developing an FDW test.

Solution: Step through using gdb, verify the field numbers passed to TupleDescAttr are within natts.

Best Practices to Avoid This Error

Pin your FDW extension version in package management to avoid silent binary changes. Always run ALTER EXTENSION UPDATE during maintenance windows.

Automate foreign table refreshes when the upstream schema changes. postgres_fdw supports IMPORT FOREIGN SCHEMA which can run in CI to detect drifts early.

Use Galaxy’s version-controlled SQL editor to store the exact CREATE FOREIGN TABLE statements.

Team members can endorse the canonical definition, reducing drift and rebuild time.

Related Errors and Solutions

fdw_invalid_option_name - occurs when an unrecognized option is passed to CREATE SERVER. Verify option spelling.

fdw_unable_to_establish_connection - indicates network or authentication failure. Check connection strings and firewall rules.

descriptor_length_mismatch (2201X) - arises when descriptor and column counts differ in EXECUTE statements. Recompile prepared statements.

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