Delete the (now redundant) product_or_parent_id_term_id index

in the product attributes lookup table.

plugins/woocommerce/src/Internal/ProductAttributesLookup/DataRegenerator.php

@@ -6,6 +6,7 @@
 namespace Automattic\WooCommerce\Internal\ProductAttributesLookup;
 
 use Automattic\WooCommerce\Internal\ProductAttributesLookup\LookupDataStore;
+use Automattic\WooCommerce\Internal\Utilities\DatabaseUtil;
 use Automattic\WooCommerce\Utilities\ArrayUtil;
 
 defined( 'ABSPATH' ) || exit;
@@ -466,27 +467,31 @@ class DataRegenerator {
  term_id bigint(20) NOT NULL,
  is_variation_attribute tinyint(1) NOT NULL,
  in_stock tinyint(1) NOT NULL,
- INDEX product_or_parent_id_term_id (product_or_parent_id, term_id),
  INDEX is_variation_attribute_term_id (is_variation_attribute, term_id),
- PRIMARY KEY  (`product_id`, `product_or_parent_id`, `taxonomy`, `term_id`)
+ PRIMARY KEY  ( `product_or_parent_id`, `term_id`, `product_id`, `taxonomy` )
 ) $collate;";
 	}
 
 	/**
 	 * Create the primary key for the table if it doesn't exist already.
+	 * It also deletes the product_or_parent_id_term_id index if it exists, since it's now redundant.
 	 *
 	 * @return void
 	 */
 	public function create_table_primary_index() {
 		global $wpdb;
 
-		// phpcs:disable WordPress.DB.PreparedSQL.InterpolatedNotPrepared
+		// phpcs:disable WordPress.DB.PreparedSQL
 		$wpdb->query(
 			"
 ALTER TABLE {$this->lookup_table_name}
-ADD PRIMARY KEY IF NOT EXISTS (`product_id`, `product_or_parent_id`, `taxonomy`, `term_id`)"
+ADD PRIMARY KEY IF NOT EXISTS ( `product_or_parent_id`, `term_id`, `product_id`, `taxonomy` )"
 		);
-		// phpcs:enable WordPress.DB.PreparedSQL.InterpolatedNotPrepared
+		// phpcs:enable WordPress.DB.PreparedSQL
+
+		wc_get_container()
+			->get( DatabaseUtil::class )
+			->drop_table_index( $this->lookup_table_name, 'product_or_parent_id_term_id' );
 	}
 
 	/**

plugins/woocommerce/src/Utilities/DatabaseUtil.php

@@ -70,4 +70,37 @@ class DatabaseUtil {
 		//phpcs:ignore WordPress.DB.PreparedSQL.InterpolatedNotPrepared
 		return $wpdb->query( "DROP TABLE IF EXISTS `{$table_name}`" );
 	}
+
+	/**
+	 * Drops a table index, if both the table and the index exist.
+	 *
+	 * @param string $table_name The name of the table that contains the index.
+	 * @param string $index_name The name of the index to be dropped.
+	 * @return bool True if the index has been dropped, false if either the table or the index don't exist.
+	 */
+	public function drop_table_index( string $table_name, string $index_name ): bool {
+		global $wpdb;
+
+		// phpcs:disable WordPress.DB.PreparedSQL
+
+		$index_count = intval(
+			$wpdb->get_var(
+				"
+SELECT COUNT(1) FROM INFORMATION_SCHEMA.STATISTICS
+WHERE table_name='$table_name'
+AND table_schema='" . DB_NAME . "'
+AND index_name='$index_name'"
+			)
+		);
+
+		if ( 0 === $index_count ) {
+			return false;
+		}
+
+		$wpdb->query( "ALTER TABLE $table_name DROP INDEX $index_name" );
+
+		// phpcs:enable WordPress.DB.PreparedSQL
+
+		return true;
+	}
 }