PostgreSQL HV010 Error: Causes and Solutions Complete Guide

#postgres #database #dba #sql

PostgreSQL Error HV010: fdw_function_sequence_error

HV010 fdw_function_sequence_error occurs when PostgreSQL's Foreign Data Wrapper (FDW) subsystem detects that its internal callback functions have been invoked in an incorrect or unexpected order. This typically surfaces during custom FDW development, extension version mismatches after major upgrades, or abnormal transaction/cursor usage involving foreign tables.

Top 3 Causes

1. FDW Callback Function Order Violation

The FDW API enforces a strict lifecycle: BeginForeignScan → IterateForeignScan → EndForeignScan. Skipping or reordering these callbacks immediately triggers HV010.

-- Check registered FDWs and their handler functions
SELECT fdwname,
       fdwhandler::regproc AS handler,
       fdwvalidator::regproc AS validator
FROM pg_foreign_data_wrapper;

-- Inspect handler function details
SELECT proname, prolang, prosrc
FROM pg_proc
WHERE proname LIKE '%fdw_handler%';

Fix: Drop and recreate the FDW with the correct handler:

-- Remove broken FDW
DROP FOREIGN DATA WRAPPER broken_fdw CASCADE;

-- Recreate with correct handler
CREATE FOREIGN DATA WRAPPER my_fdw
  HANDLER my_fdw_handler
  VALIDATOR my_fdw_validator;

-- Re-register the foreign server
CREATE SERVER my_remote_server
  FOREIGN DATA WRAPPER my_fdw
  OPTIONS (host '10.0.0.1', port '5432', dbname 'targetdb');

2. Extension Version Mismatch After Upgrade

After a PostgreSQL major version upgrade, if ALTER EXTENSION ... UPDATE is not run, the shared library (.so) and SQL catalog may be out of sync, causing internal symbol conflicts that produce HV010.

-- Detect outdated FDW extensions
SELECT name,
       installed_version,
       default_version,
       CASE
           WHEN installed_version <> default_version THEN 'NEEDS UPDATE'
           ELSE 'OK'
       END AS status
FROM pg_available_extensions
WHERE name LIKE '%fdw%'
  AND installed_version IS NOT NULL;

-- Update the extension
ALTER EXTENSION postgres_fdw UPDATE;

-- If corrupted, reinstall completely
DROP EXTENSION postgres_fdw CASCADE;
CREATE EXTENSION postgres_fdw;

3. Abnormal Transaction or Cursor Usage

Using ROLLBACK TO SAVEPOINT mid-scan or closing a cursor before completing a fetch cycle can leave the FDW state machine in an inconsistent state, triggering HV010.

-- Find sessions stuck in FDW-related idle transactions
SELECT pid, usename, state, query, now() - state_change AS idle_duration
FROM pg_stat_activity
WHERE state = 'idle in transaction'
  AND query ILIKE '%foreign%';

-- Terminate stuck sessions if necessary
SELECT pg_terminate_backend(pid)
FROM pg_stat_activity
WHERE state = 'idle in transaction'
  AND query ILIKE '%foreign%'
  AND pid <> pg_backend_pid();

-- Use safe transaction pattern with FDW
BEGIN;
SAVEPOINT safe_point;

DO $$
DECLARE r RECORD;
BEGIN
    FOR r IN SELECT * FROM my_foreign_table LOOP
        RAISE NOTICE '%', r;
    END LOOP;
EXCEPTION
    WHEN OTHERS THEN
        ROLLBACK TO SAVEPOINT safe_point;
        RAISE;
END;
$$;

COMMIT;

Quick Fix Checklist

Verify FDW extension versions — run ALTER EXTENSION <fdw_name> UPDATE after every major PostgreSQL upgrade.
Drop and recreate broken FDWs — use CASCADE to clean up dependent foreign servers, user mappings, and foreign tables.
Kill idle-in-transaction sessions — use pg_terminate_backend() to release stuck FDW state machines.
Check foreign server connectivity — validate options with SELECT * FROM pg_foreign_server and test with a simple SELECT ... LIMIT 1.

Prevention Tips

Automate version checks: Add a monitoring query to your alerting system that flags any installed FDW extension where installed_version <> default_version.
Wrap all FDW queries in exception handlers: Any PL/pgSQL routine that touches a foreign table should include a proper EXCEPTION WHEN OTHERS block to prevent state corruption from propagating.

-- Reusable FDW health check
CREATE OR REPLACE FUNCTION fdw_server_exists(p_server TEXT)
RETURNS BOOLEAN LANGUAGE plpgsql AS $$
BEGIN
    PERFORM 1 FROM pg_foreign_server WHERE srvname = p_server;
    RETURN FOUND;
EXCEPTION
    WHEN OTHERS THEN RETURN FALSE;
END;
$$;

-- Usage
SELECT fdw_server_exists('my_remote_server');

Related Error Codes

Code	Name	Notes
HV000	fdw_error	Generic FDW error, parent class of HV010
HV005	fdw_column_name_not_found	Column mapping mismatch in foreign table
HV021	fdw_inconsistent_descriptor_information	Schema mismatch between remote and local definition
08001	sqlclient_unable_to_establish_sqlconnection	Connection-level failure, check before HV010

📖 Want a more detailed guide?
Check out the full in-depth version (Korean) on oraerror.com — includes detailed analysis, additional SQL examples, and prevention tips.

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