PDOException: SQLSTATE[HY000]: General error: 2014 Cannot execute queries while other unbuffered queries are active. Consider using PDOStatement::fetchAll(). Alternatively, if your code is only ever going to run against mysql, you may enable query buffering by setting the PDO::MYSQL_ATTR_USE_BUFFERED_QUERY attribute.
呃，上面的错误是什么东西？
举个栗子来说，如下的写法可能导致上述错误的发生：
$connection = new \PDO('mysql:dbname=test;host=127.0.0.1', 'username', 'password'); $stmt_one = $connection->prepare('SQL ONE'); $stmt_one->execute(); $stmt_two = $connection->prepare('SQL TWO'); $stmt_two->execute(); $stmt_two->fetchAll(PDO::FETCH_ASSOC); |
也就是说，在同一个请求中使用了多个活跃的 PDOStatement 来操作数据时，有可能会遇到这个问题。
说可能是因为我遇到的情况是某些环境下没有问题，某些环境会出问题。嗯，总得来说是姿势不对。
按照错误中提示的修复方式经测试不是很管用，不管是在 prepare() 时指定 PDO::MYSQL_ATTR_USE_BUFFERED_QUERY 属性，或者是注意 fetchAll()。
其实正确的解决办法网上已经有不少地方说明了，示例如下：
$stmt_one = $connection->prepare('SQL ONE'); $stmt_one->execute(); $stmt_one->closeCursor(); $stmt_two = $connection->prepare('SQL TWO'); $stmt_two->execute(); $stmt_two->fetchAll(PDO::FETCH_ASSOC); $stmt_two->closeCursor(); |
即在使用完毕 PDOStatement 之后，使用 closeCursor 方法关闭游标。
关闭游标之后，该 PDOStatement 还能继续使用吗？
是的，是可以继续使用的：
PDOStatement::closeCursor() frees up the connection to the server so that other SQL statements may be issued, but leaves the statement in a state that enables it to be executed again.
This method is useful for database drivers that do not support executing a PDOStatement object when a previously executed PDOStatement object still has unfetched rows. If your database driver suffers from this limitation, the problem may manifest itself in an out-of-sequence error.
PDOStatement::closeCursor() is implemented either as an optional driver specific method (allowing for maximum efficiency), or as the generic PDO fallback if no driver specific function is installed.
其实文档中也解释了为什么有些情况下没问题，有些情况下有问题，是数据库驱动的支持情况不同导致的。
