PostgreSQL 22026 Error: Causes and Solutions Complete Guide

PostgreSQL Error 22026: String Data Length Mismatch

PostgreSQL error code 22026 (string_data_length_mismatch) occurs when the actual length of string or binary data does not match the expected length during data processing or type conversion. This error commonly surfaces when working with bytea data types, fixed-length char(n) columns, or custom type serialization/deserialization routines. Understanding the root cause quickly is essential, as this error can block data pipelines and integrations if left unresolved.

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Top 3 Causes

1. Invalid bytea Encoding or Malformed Hex String

When inserting or converting bytea data, the input hex string must have an even number of hexadecimal digits. If the hex string has an odd number of characters or contains invalid characters, PostgreSQL cannot correctly determine the byte length, triggering error 22026.

-- BAD: Odd number of hex digits causes an error
SELECT decode('A1B2C', 'hex');
-- ERROR: invalid hexadecimal data: odd number of digits

-- GOOD: Even number of hex digits
SELECT decode('A1B2C3', 'hex');

-- Safe insertion of bytea data
INSERT INTO file_storage (file_id, file_data)
VALUES (1, decode('DEADBEEF01', 'hex'));

-- Verify stored bytea length
SELECT file_id, octet_length(file_data) AS bytes
FROM file_storage;

2. Fixed-Length char(n) Type Mismatch During Custom Casting

The char(n) type enforces a strict fixed-length storage contract. When custom cast functions or type conversion routines produce output that does not match the declared length n, the internal length validation fails and raises 22026. This frequently occurs in legacy data migrations or ETL pipelines where character encodings are mixed.

-- Check column definitions for fixed-length types
SELECT column_name, data_type, character_maximum_length
FROM information_schema.columns
WHERE table_name = 'your_table'
  AND data_type = 'character';

-- Safely pad input to match char(n) length
INSERT INTO product_codes (code)
VALUES (lpad('ABC', 10, ' '));

-- If mismatches are frequent, consider migrating to varchar
ALTER TABLE product_codes
    ALTER COLUMN code TYPE varchar(10);

-- Inspect length distribution of existing data
SELECT char_length(code) AS len, COUNT(*) AS cnt
FROM product_codes
GROUP BY char_length(code)
ORDER BY len;

3. Custom or Composite Type Binary Layout Mismatch

When a user-defined type (UDT) or composite type is modified — especially after an extension upgrade — the binary layout stored on disk may no longer match what the new type definition expects. Reading old binary data through a new type definition fails at the length validation step, producing error 22026. This is the trickiest cause to diagnose and can risk data integrity.

-- Inspect custom type metadata
SELECT typname, typlen, typbyval, typsend, typreceive
FROM pg_type
WHERE typname = 'your_custom_type';

-- Back up data as text before modifying the type
CREATE TABLE backup_typed_data AS
SELECT id, your_col::text AS your_col_text
FROM your_table;

-- After redefining the type, restore with explicit cast
INSERT INTO your_table (id, your_col)
SELECT id, your_col_text::your_custom_type
FROM backup_typed_data;

-- Check installed extension versions
SELECT extname, extversion
FROM pg_extension;

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Quick Fix Solutions

-- Validate hex strings before inserting bytea
CREATE OR REPLACE FUNCTION is_valid_hex(input text)
RETURNS boolean AS $$
BEGIN
    RETURN length(input) % 2 = 0 AND input ~ '^[0-9a-fA-F]*$';
END;
$$ LANGUAGE plpgsql IMMUTABLE;

-- Use this function as a CHECK constraint
ALTER TABLE file_storage
ADD CONSTRAINT chk_valid_hex
CHECK (is_valid_hex(encode(file_data, 'hex')));

-- Safely convert and verify bytea round-trip
SELECT
    original_hex,
    encode(decode(original_hex, 'hex'), 'hex') AS round_trip_hex,
    original_hex = encode(decode(original_hex, 'hex'), 'hex') AS is_valid
FROM (VALUES ('DEADBEEF'), ('A1B2C3')) AS t(original_hex);

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Prevention Tips

1. Validate data length and format at the application layer before sending it to PostgreSQL. For bytea columns, always ensure hex strings have an even character count. For char(n) columns, explicitly pad or truncate strings to the correct length before insertion. Adding CHECK constraints at the database level provides an additional safety net.

2. Test type changes in a staging environment before production deployment. Whenever you modify a custom type, composite type, or upgrade an extension, run pg_upgrade --check for major version upgrades and always back up affected data as text before applying schema changes. Keeping a clear versioning strategy for custom types prevents binary layout conflicts from ever reaching production.

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Related Errors

Code	Name	Brief Description
22001	string_data_right_truncation	String exceeds target column length
22P02	invalid_text_representation	Invalid text format for type conversion
42804	datatype_mismatch	Type-level mismatch (vs. length-level for 22026)
22003	numeric_value_out_of_range	Numeric value exceeds type range during conversion

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