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PostgreSQL HV090 Error: Causes and Solutions Complete Guide

#postgres #database #dba #sql

PostgreSQL HV090: fdw_invalid_string_length_or_buffer_length

PostgreSQL error code HV090 (fdw_invalid_string_length_or_buffer_length) occurs when a Foreign Data Wrapper (FDW) encounters a mismatch between the defined string or buffer length and the actual data being transferred from a remote source. This error is common with drivers like postgres_fdw, oracle_fdw, and mysql_fdw, and it can silently break production data pipelines if left unaddressed.

Top 3 Causes and Fixes

1. Column Length Mismatch Between Foreign Table and Remote Source

This is the most frequent cause. When the foreign table definition uses a shorter VARCHAR length than what the remote server actually stores, any row exceeding that length triggers HV090.

-- Problematic definition (remote column is VARCHAR(500), but defined as VARCHAR(50))
CREATE FOREIGN TABLE foreign_users (
    id    INT,
    name  VARCHAR(50)   -- Too short! Remote DB has VARCHAR(500)
)
SERVER remote_pg_server
OPTIONS (schema_name 'public', table_name 'users');

-- Fix: Use IMPORT FOREIGN SCHEMA to auto-sync the remote schema
DROP SCHEMA IF EXISTS remote_schema CASCADE;
CREATE SCHEMA remote_schema;

IMPORT FOREIGN SCHEMA public
    LIMIT TO (users)
    FROM SERVER remote_pg_server
    INTO remote_schema;

-- Or manually correct the column definition
ALTER FOREIGN TABLE foreign_users
    ALTER COLUMN name TYPE TEXT;  -- Use TEXT when max length is uncertain
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2. Multibyte Character Encoding Overflow

When a VARCHAR(n) column is defined with a character limit, multibyte characters (e.g., UTF-8 Korean, Japanese, Chinese) consume 2–4 bytes per character. A VARCHAR(100) column can overflow its buffer when storing 100 multibyte characters that actually require 300+ bytes internally.

-- Check actual byte length vs character length on a foreign table
SELECT
    id,
    LENGTH(description)        AS char_length,
    OCTET_LENGTH(description)  AS byte_length
FROM foreign_products
WHERE OCTET_LENGTH(description) > 200
LIMIT 20;

-- Fix: Switch VARCHAR columns to TEXT for multibyte-safe handling
DROP FOREIGN TABLE IF EXISTS foreign_products;

CREATE FOREIGN TABLE foreign_products (
    id          INT,
    name        TEXT,           -- Safe for any multibyte content
    description TEXT
)
SERVER remote_server
OPTIONS (schema_name 'public', table_name 'products');
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3. FDW Driver Buffer/Option Misconfiguration

Some FDW drivers expose options like max_blob_size or fetch_size that control internal buffer sizes. If these are set too low — or left at defaults that no longer fit your data volume — HV090 can appear as your data grows over time.

-- Check current server options
SELECT srvname, srvoptions
FROM pg_foreign_server
WHERE srvname = 'remote_oracle_server';

-- Fix for oracle_fdw: increase max_blob_size (default is 512KB)
ALTER SERVER remote_oracle_server
OPTIONS (SET max_blob_size '10485760');  -- Set to 10MB

-- Fix for postgres_fdw: reduce fetch_size to lower memory pressure
ALTER SERVER remote_pg_server
OPTIONS (SET fetch_size '100');

-- Apply per-table fetch_size for high-volume foreign tables
ALTER FOREIGN TABLE foreign_large_orders
OPTIONS (SET fetch_size '50');

-- Verify the fix
SELECT COUNT(*) FROM foreign_large_orders;
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Quick Fix Checklist

  1. Always prefer TEXT over VARCHAR(n) for foreign table columns when the remote column size is unknown or variable.
  2. Use IMPORT FOREIGN SCHEMA instead of manually writing CREATE FOREIGN TABLE — it mirrors the remote definition exactly.
  3. Increase FDW buffer options (max_blob_size, fetch_size) if your data involves LOBs or large text fields.
  4. Check encoding: ensure client, server, and remote database encodings are consistent (SHOW server_encoding;).

Prevention Tips

Automate schema synchronization. Remote schemas change — columns get extended, types change. Schedule a periodic IMPORT FOREIGN SCHEMA refresh (via pg_cron or a CI pipeline) so your foreign table definitions never fall out of sync.

Monitor column length utilization. Run a regular check to catch columns approaching their defined limit before they cause failures in production:

-- Alert when any value exceeds 80% of the defined VARCHAR limit
SELECT
    MAX(LENGTH(name))                        AS max_observed,
    200                                      AS defined_limit,
    ROUND(MAX(LENGTH(name))::NUMERIC / 200 * 100, 1) AS pct_used
FROM foreign_users
HAVING MAX(LENGTH(name)) > 200 * 0.8;
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Related Errors

Code Name Relation
HV000 fdw_error Parent error class for all FDW errors
HV005 fdw_column_name_not_found Schema drift, often co-occurs with HV090
22001 string_data_right_truncation Similar root cause in non-FDW contexts
HV002 fdw_dynamic_parameter_value_needed Bad FDW option config, related to cause #3

📖 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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