fix(MySQL): combine the drop and the creation of an index over the same columns

[fballiano]

Q	A
Type	bug
Fixed issues	n/a

Summary

AbstractMySQLPlatform combines the drop of an index and the creation of another one over the same columns into a single ALTER TABLE ... DROP INDEX a, ADD INDEX b (...) statement, so that the columns are never left unindexed in between. The two column lists are compared through the legacy Index::getColumns() accessor, which returns the raw map keys: the introspection API marks them as quoted ('"parent_id"'), while an index built in memory keeps them bare ('parent_id'). They therefore never match when the dropped index comes from an introspected table, the statements are not combined, and the drop is emitted on its own.

That makes alterTable() fail whenever the dropped index is the one backing a foreign key. InnoDB rejects the drop even though the added index would provide the same cover, and disabling foreign_key_checks does not lift the restriction:

$conn->executeStatement('CREATE TABLE parent (id INT NOT NULL, PRIMARY KEY (id)) ENGINE = InnoDB');
$conn->executeStatement(<<<'SQL'
    CREATE TABLE child (
        id INT NOT NULL,
        parent_id INT NOT NULL,
        val INT NOT NULL,
        PRIMARY KEY (id),
        UNIQUE KEY uniq_parent_val (parent_id, val),
        CONSTRAINT fk_child_parent FOREIGN KEY (parent_id) REFERENCES parent (id)
    ) ENGINE = InnoDB
SQL);

$sm  = $conn->createSchemaManager();
$old = $sm->introspectTableByUnquotedName('child');

// Redeclare the unique key as a plain index over the same columns.
$new = $old->edit()
    ->setIndexes(
        Index::editor()
            ->setUnquotedName('idx_parent_val')
            ->setUnquotedColumnNames('parent_id', 'val')
            ->create(),
    )
    ->create();

$sm->alterTable($sm->createComparator()->compareTables($old, $new));
// SQLSTATE[HY000]: General error: 1553 Cannot drop index 'uniq_parent_val':
// needed in a foreign key constraint

Emitted before the fix, where the drop runs on its own and never reaches the creation:

DROP INDEX `uniq_parent_val` ON child;
CREATE INDEX idx_parent_val ON child (parent_id, val);

and after:

ALTER TABLE child DROP INDEX uniq_parent_val, ADD INDEX idx_parent_val (parent_id, val);

Fix

Read the column names through the non-deprecated Index::getIndexedColumns() accessor and compare their unquoted identifier values, so the comparison is agnostic of whether the index was introspected or built in memory. This is the same treatment #7392 applied to the same accessor in the SQLite table-recreation path.

The regression test goes to MySQLSchemaManagerTest: the combining is MySQL-specific, and so is the foreign-key index requirement that makes the uncombined form fail. It errors with 1553 on 4.4.x and passes with the fix.

[morozov]

@fballiano thank you for the patch. Please see inline comments.

[fballiano]

Hi @morozov, thank you for the review and sorry for the mistakes, I've rewritten the whole thing with your suggestions, let's see if CI agrees.

🙏

[fballiano]

it was so easy 🤦🏻 sorry 😅

[morozov]

Merged. Thanks, @fballiano!