PostgreSQL Error - HV001 fdw_out_of_memory Error: Causes and Fixes

What is the fdw_out_of_memory error in PostgreSQL?

fdw_out_of_memory is a SQLSTATE HV001 error raised by PostgreSQL’s foreign data wrapper (FDW) subsystem when it cannot allocate additional memory for a remote scan, modify, or join operation.

The backend cancels the statement after failing an internal palloc call, protecting itself from swapping or crashing.

Fixing the error is critical because the query terminates immediately and downstream applications receive an exception.

What Causes This Error?

Large result sets coming from a foreign table can exceed the process memory governed by work_mem and maintenance_work_mem.

Hash joins or sorts performed locally on foreign rows may allocate huge temporary structures and trigger the failure.

FDWs that buffer the entire remote result (postgres_fdw, mysql_fdw) amplify memory consumption, especially when fetch_size is high.

Connection pooling settings that share backends with many active cursors can raise the baseline memory footprint before the FDW work even begins.

How to Fix fdw_out_of_memory

First, confirm the failing statement with EXPLAIN ANALYZE to see where memory spikes.

Add LIMIT, WHERE, or JOIN filters on the foreign server to shrink the dataset.

Increase work_mem for the session or role when filtering is not possible. Trade memory for disk by enabling enable_hashjoin=off or enable_sort=off so PostgreSQL externalizes those operations.

Tune FDW parameters: lower fetch_size, disable batch insert size, or switch to cursor-based fetching if the extension supports it.

Common Scenarios and Solutions

Analytics queries that join large remote fact tables often hit the limit.

Push the aggregation to the remote server using a view or foreign server side function.

ETL jobs loading millions of rows via INSERT INTO local_table SELECT * FROM foreign_table can fail midway. Insert in smaller batches or COPY the data on the remote side.

Best Practices to Avoid This Error

Always project only required columns and filter early in the foreign query to minimize transferred bytes.

Set realistic per user work_mem limits and monitor pg_stat_activity for memory usage spikes.

Use pg_stat_kcache or auto_explain to detect heavy FDW allocations.

Galaxy’s SQL editor highlights excessive SELECT * patterns and suggests LIMIT clauses, helping teams prevent runaway memory use before execution.

Related Errors and Solutions

HV00N: fdw_invalid_handle – indicates stale cursor handle and may appear after an out of memory abort. Reopen the connection.

53200: out_of_memory – generic allocator failure that can occur outside FDW. Apply similar memory tuning steps.

54000: program_limit_exceeded – triggered by excessively deep expression trees. Simplify the query.

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