PostgreSQL Error - HV00M fdw_unable_to_create_reply Error Explained and Fixed

What is fdw_unable_to_create_reply and how do I fix it?

PostgreSQL throws fdw_unable_to_create_reply (SQLSTATE HV00M) when a foreign data wrapper (FDW) fails while constructing the result set returned from a remote data source.

The server aborts the query before any rows reach the client, so downstream applications will see an immediate error.

Fixing this fault is critical because it blocks access to external data mapped through FOREIGN TABLEs.

What Causes This Error?

Insufficient work_mem or overall RAM can prevent the FDW from allocating space for the incoming tuple batch, which triggers the error quickly on large result sets.

Unstable or slow network links may produce partial or corrupt packets, leaving the FDW unable to form a valid reply structure.

Incorrect FDW options, such as an excessively large fetch_size or row limit, can overwhelm buffers and provoke the failure.

Bugs in the specific FDW extension version can surface under heavy load, causing reply creation to abort.

How to Fix fdw_unable_to_create_reply

First, raise memory allowances: SET work_mem TO '128MB'; then rerun the query to confirm the error disappears.

If memory is sufficient, lower the fetch_size option on the affected FOREIGN TABLE so the FDW requests fewer rows per round-trip.

Update the FDW extension to the latest stable release and restart PostgreSQL to pick up bug fixes.

Verify network stability with ping or traceroute and schedule heavy FDW workloads during low-latency windows.

Common Scenarios and Solutions

Large analytical joins against foreign tables frequently hit the error.

Reducing query result width or adding WHERE clauses to push filters down avoids reply bloat.

ETL jobs pulling millions of rows can exhaust memory.

Switching to cursor-based fetching or batching by key range resolves the issue.

Best Practices to Avoid This Error

Right-size work_mem and shared_buffers based on expected FDW workloads, and monitor them with pg_stat_activity.

Define realistic fetch_size values (1000-5000) on each FOREIGN TABLE instead of relying on defaults.

Regularly update FDW extensions and PostgreSQL itself to incorporate performance and stability fixes.

Related Errors and Solutions

HV00L fdw_too_many_handles appears when too many connections remain open; close idle connections or raise the FDW handle limit.

HV00P fdw_reply_handle_unknown indicates the remote server sent an unexpected protocol token; ensure version compatibility between servers.

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