Source

File: wp-includes/wp-db.php

protected function process_field_charsets( $data, $table ) {
		foreach ( $data as $field => $value ) {
			if ( '%d' === $value['format'] || '%f' === $value['format'] ) {
				/*
				 * We can skip this field if we know it isn't a string.
				 * This checks %d/%f versus ! %s because its sprintf() could take more.
				 */
				$value['charset'] = false;
			} else {
				$value['charset'] = $this->get_col_charset( $table, $field );
				if ( is_wp_error( $value['charset'] ) ) {
					return false;
				}
			}

			$data[ $field ] = $value;
		}

		return $data;
	}