EC
1594
MariaDBERRORNotableSchemaHIGH confidence

Cannot drop index: needed by a foreign key constraint

Production Risk

Low — the DROP INDEX fails; referential integrity is preserved.

What this means

An index that supports a foreign key constraint cannot be dropped without first removing the constraint.

Why it happens
  1. 1Attempting to DROP INDEX on an index that is used by a foreign key on the same or a referencing table.
How to reproduce
trigger — this will error
trigger — this will error
ALTER TABLE orders DROP INDEX idx_customer_id;
-- when a FK uses this index

expected output

ERROR 1594 (HY000): Cannot drop index 'idx_customer_id': needed in a foreign key constraint

Fix

Drop the foreign key first

Drop the foreign key first
ALTER TABLE orders DROP FOREIGN KEY fk_customer;
ALTER TABLE orders DROP INDEX idx_customer_id;

Why this works

Foreign key constraints depend on supporting indexes; drop the constraint before the index.

Sources
Official documentation ↗

MySQL 8.0 — 1594 ER_DROP_INDEX_FK

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