在模式升级前运行。

function pre_schema_upgrade() {
	global $wp_current_db_version, $wpdb;

	// Upgrade versions prior to 2.9.
	if ( $wp_current_db_version query( "DELETE o1 FROM $wpdb->options AS o1 JOIN $wpdb->options AS o2 USING (`option_name`) WHERE o2.option_id > o1.option_id" );

		// Drop the old primary key and add the new.
		$wpdb->query( "ALTER TABLE $wpdb->options DROP PRIMARY KEY, ADD PRIMARY KEY(option_id)" );

		// Drop the old option_name index. dbDelta() doesn't do the drop.
		$wpdb->query( "ALTER TABLE $wpdb->options DROP INDEX option_name" );
	}

	// Multisite schema upgrades.
	if ( $wp_current_db_version query( "ALTER TABLE $wpdb->signups ADD signup_id BIGINT(20) NOT NULL AUTO_INCREMENT PRIMARY KEY FIRST" );
			$wpdb->query( "ALTER TABLE $wpdb->signups DROP INDEX domain" );
		}

		if ( $wp_current_db_version query( "ALTER TABLE $wpdb->blogs CHANGE COLUMN archived archived varchar(1) NOT NULL default '0'" );
			$wpdb->query( "ALTER TABLE $wpdb->blogs CHANGE COLUMN archived archived tinyint(2) NOT NULL default 0" );
		}
	}

	// Upgrade versions prior to 4.2.
	if ( $wp_current_db_version query( "ALTER TABLE $wpdb->usermeta DROP INDEX meta_key, ADD INDEX meta_key(meta_key(191))" );
		}
		$wpdb->query( "ALTER TABLE $wpdb->terms DROP INDEX slug, ADD INDEX slug(slug(191))" );
		$wpdb->query( "ALTER TABLE $wpdb->terms DROP INDEX name, ADD INDEX name(name(191))" );
		$wpdb->query( "ALTER TABLE $wpdb->commentmeta DROP INDEX meta_key, ADD INDEX meta_key(meta_key(191))" );
		$wpdb->query( "ALTER TABLE $wpdb->postmeta DROP INDEX meta_key, ADD INDEX meta_key(meta_key(191))" );
		$wpdb->query( "ALTER TABLE $wpdb->posts DROP INDEX post_name, ADD INDEX post_name(post_name(191))" );
	}

	// Upgrade versions prior to 4.4.
	if ( $wp_current_db_version get_var( "SHOW TABLES LIKE '{$wpdb->termmeta}'" ) && $wpdb->get_results( "SHOW INDEX FROM {$wpdb->termmeta} WHERE Column_name = 'meta_key'" ) ) {
			$wpdb->query( "ALTER TABLE $wpdb->termmeta DROP INDEX meta_key, ADD INDEX meta_key(meta_key(191))" );
			maybe_convert_table_to_utf8mb4( $wpdb->termmeta );
		}
	}
}