<p>The COALESCE PARTITION command was run on a table that is not partitioned by HASH or KEY, which MySQL does not allow.</p>
<p>MySQL Error 1509: ER_COALESCE_ONLY_ON_HASH_PARTITION occurs when you execute ALTER TABLE ... COALESCE PARTITION on a table partitioned by RANGE, LIST, or other methods. The fix is to use COALESCE only on tables partitioned by HASH or KEY, or switch to ALTER TABLE ... DROP PARTITION instead.</p>
COALESCE PARTITION can only be used on HASH/KEY
MySQL throws error 1509 when ALTER TABLE ... COALESCE PARTITION is run on a table that is not partitioned by HASH or KEY. The statement tries to merge multiple partitions into fewer ones, but MySQL restricts this operation to HASH or KEY partitioning schemes.
The error text reads: "COALESCE PARTITION can only be used on HASH/KEY". It signals that the requested partition merge is incompatible with the table’s current partitioning type.
The error surfaces during table maintenance or refactoring tasks, typically in production environments where partitioning strategies evolve. It arises immediately after the COALESCE PARTITION clause is parsed and the server verifies the table’s partition method.
Leaving the issue unresolved blocks schema changes, freezes deployment pipelines, and can lead to storage bloat if redundant partitions remain. Fixing the error ensures smooth schema evolution and optimal query performance.
The primary trigger is executing COALESCE PARTITION on a RANGE, LIST, or composite partitioned table. MySQL enforces this limitation to avoid data redistribution problems specific to non-hash partition types.
Another cause is mistakenly assuming a table uses HASH partitioning when it actually uses RANGE or LIST, often after historic schema changes or imports.
First, confirm the partitioning method with SHOW CREATE TABLE or INFORMATION_SCHEMA.PARTITIONS. If the table is HASH or KEY partitioned, you can safely re-run the COALESCE statement.
If the table is RANGE or LIST partitioned, replace COALESCE PARTITION with DROP PARTITION or REORGANIZE PARTITION, which are allowed for those schemes.
In development environments, engineers often refactor a RANGE-partitioned time-series table. Switching to DROP PARTITION eliminates obsolete ranges without triggering error 1509.
In legacy imports, a table originally created with HASH partitioning may have been altered to RANGE. Re-checking the partitioning method prevents accidental COALESCE calls.
Document table partitioning strategies in version control. Use descriptive comments or a schema registry so engineers know the exact partition type before running maintenance SQL.
Automate partition management using tools like Galaxy’s task scheduler, which can validate partition types and pick the correct ALTER statement programmatically.
ERR_DROP_ONLY_ON_HASH_PARTITION arises when attempting DROP PARTITION on HASH tables - use COALESCE instead. ER_PARTITION_FUNCTION_NOT_ALLOWED signals that a non-deterministic function was used in a partition expression - rewrite the expression to a deterministic one.
Developers often try to merge old RANGE partitions; MySQL blocks this operation, raising error 1509.
Automated maintenance jobs written for HASH partitions run against tables that have since changed to LIST or RANGE.
Schema documentation drift leads engineers to believe a table is HASH partitioned when it is not.
Raised when DROP PARTITION is executed on a HASH-partitioned table. Use COALESCE PARTITION instead.
Occurs when a non-deterministic function is used in a partition expression. Replace with a deterministic function.
Happens when attempting REORGANIZE on partitions that do not exist. Verify partition names first.
No. MySQL’s partitioning engine prohibits it. Use DROP or REORGANIZE instead.
No data is lost, but MySQL redistributes rows across new partitions. Always test on backups first.
Galaxy’s AI copilot inspects SHOW CREATE TABLE output, warns if COALESCE is used on an unsupported partition type, and suggests the correct ALTER statement.
On HASH tables, COALESCE is optimized and often faster. On non-hash tables, DROP PARTITION is the only valid option.
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