Redundant foreign-key backing index when a unique constraint covers the same columns

[morozov]

DBAL creates a backing index for a foreign key even when a unique constraint already covers the referencing columns.

The implicit index is a workaround for schema comparison. It mirrors the backing index MySQL and MariaDB create automatically for a foreign key. When a unique constraint covers the columns, those engines create no such index — they reuse the unique one instead. So the implicit index mirrors nothing, and it duplicates the unique constraint's index. It is redundant twice over.

Reproduce

$child = Table::editor()
    ->setUnquotedName('child')
    ->setColumns(
        Column::editor()
            ->setUnquotedName('id')
            ->setTypeName(Types::INTEGER)
            ->create(),
        Column::editor()
            ->setUnquotedName('parent_id')
            ->setTypeName(Types::INTEGER)
            ->create(),
    )
    ->setUniqueConstraints(
        UniqueConstraint::editor()
            ->setUnquotedColumnNames('parent_id')
            ->create(),
    )
    ->setForeignKeyConstraints(
        ForeignKeyConstraint::editor()
            ->setUnquotedReferencingColumnNames('parent_id')
            ->setUnquotedReferencedTableName('parent')
            ->setUnquotedReferencedColumnNames('id')
            ->create(),
    )
    ->create();

echo implode("\n", (new MySQLPlatform())->getCreateTableSQL($child));

The code above produces:

CREATE TABLE child (
    id        INT NOT NULL,
    parent_id INT NOT NULL,
    UNIQUE (parent_id),
    INDEX     IDX_22B35429727ACA70 (parent_id)
)
ALTER TABLE child
    ADD FOREIGN KEY (parent_id) REFERENCES parent (id)

The implicit INDEX IDX_22B35429727ACA70 (parent_id) is redundant — the explicit UNIQUE (parent_id) already covers that column.