EC
1495
MariaDBERRORNotablePartitioningHIGH confidence

List of fields is only allowed in KEY partitions

Production Risk

Low — DDL error; the table will not be created.

What this means

ER_LIST_OF_FIELDS_ONLY_IN_HASH_ERROR (1495, SQLSTATE HY000) is raised when a list of columns is used in a HASH partition definition, which is not supported.

Why it happens
  1. 1PARTITION BY HASH(col1, col2) — multiple columns are not valid for HASH
  2. 2Confusion between HASH (single expression) and KEY (multiple columns) partitioning
How to reproduce
trigger — this will error
trigger — this will error
CREATE TABLE t (id INT, region VARCHAR(50))
PARTITION BY HASH(id, region)  -- Multiple cols not valid for HASH
PARTITIONS 4;

expected output

ERROR 1495 (HY000): List of fields is only allowed in KEY partitions

Fix

Use KEY partitioning for multiple columns or a single expression for HASH

Use KEY partitioning for multiple columns or a single expression for HASH
-- Single expression for HASH:
CREATE TABLE t (id INT, region VARCHAR(50))
PARTITION BY HASH(id)
PARTITIONS 4;

-- Multiple columns with KEY:
CREATE TABLE t (id INT, region VARCHAR(50))
PARTITION BY KEY(id, region)
PARTITIONS 4;

Why this works

HASH partitioning accepts a single integer expression; KEY partitioning accepts a list of column names.

Sources
Official documentation ↗

MySQL 8.0 — 1495 ER_LIST_OF_FIELDS_ONLY_IN_HASH_ERROR

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