<p>MySQL raises error 1655 when a RANGE, LIST or KEY partitioning function references more than the allowed 16 columns.</p>
<p>MySQL Error 1655: ER_TOO_MANY_PARTITION_FUNC_FIELDS_ERROR happens when a partition function or KEY partition lists over 16 columns. Reduce the column count or create a composite column and repartition the table to resolve the error.</p>
Too many fields in '%s'
MySQL throws error 1655 with the message Too many fields in '%s' when a partitioning expression or KEY partition includes more than 16 columns. Partitioning metadata is limited to 16 fields, and exceeding that limit violates internal storage rules.
The error arises during CREATE TABLE, ALTER TABLE or OPTIMIZE TABLE statements that define or redefine partitioning. It blocks execution to protect the storage engine from an unmanageable partition descriptor.
Excessive columns listed in PARTITION BY KEY(...), RANGE(...), LIST(...) clauses directly trigger the error. It can also occur when MySQL implicitly adds hidden primary key columns generated by InnoDB.
Using functional key parts (e.g., YEAR(date_col), TO_DAYS(date_col)) counts each referenced column toward the 16 column limit, even if wrapped in a function.
Reduce the number of columns in the partition function or KEY clause to sixteen or fewer. Where logical, combine multiple columns into a single generated column and partition on that instead.
For KEY partitioning, consider hashing a subset of the most selective columns only, or switch to RANGE partitioning on a pre-combined key field.
Large composite primary keys often create this error when copied verbatim into PARTITION BY KEY. Drop low-cardinality columns from the partition list to stay under the limit.
Data-warehouse tables that partition by year, month, day and additional dimensions can exceed the limit. Use a DATE or DATETIME surrogate column and partition by RANGE on that single field.
Plan partitioning during schema design and keep the key concise. Document the 16-column limit in engineering guidelines and add automated lint checks in Galaxy or CI pipelines.
Use generated columns to encapsulate multi-field logic and partition by the generated value. Monitor failed DDL statements in Galaxy to catch violations early.
Error 1491 (ER_PARTITION_FUNC_NOT_ALLOWED): Raised when non-deterministic functions appear in a partition expression. Use deterministic functions or stored columns.
Error 1503 (ER_PARTITION_COLUMN_ERROR): Triggered when the partition column is not part of every UNIQUE key. Include the column in all unique indexes to fix.
The partition clause explicitly names over sixteen columns.
InnoDB adds a 6-byte hidden PK when no explicit primary key exists, pushing the count over the limit.
Each column referenced inside expressions counts toward the limit, not just the function itself.
Non-deterministic or context-specific functions in partition expressions. Replace the function with deterministic alternatives.
Partition column not included in all UNIQUE keys. Extend indexes to include the partition column.
Column referenced in partition clause does not exist. Correct the column name or add the column.
Yes. Generated columns count toward the 16 column ceiling when referenced in the partition clause.
No. The limit is hard-coded in MySQL storage structures and cannot be changed at runtime.
Index removal does not affect partition clause validation. Only reducing columns in the partition expression resolves the error.
Galaxy highlights DDL errors instantly and suggests reducing partition columns or using generated fields, preventing wasted deploy cycles.
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