How to Solve the MySQL Error 'Cannot execute queries while other unbuffered queries are active'?

Addressing "Cannot execute queries while other unbuffered queries are active" Error with MySQL

Introduction:

MySQL's client protocol imposes a limitation where only one query can be in execution concurrently. When attempting to execute additional queries while results from previous queries remain unfetched, users may encounter the error "Cannot execute queries while other unbuffered queries are active." This issue can arise in specific scenarios, particularly when PHP Data Objects (PDO) is used with the PDO::ATTR_EMULATE_PREPARES attribute set to false.

Cause:

The root cause lies in the MySQL client protocol's inability to have multiple queries in progress simultaneously. Once a query is executed, the client library automatically fetches all its results and caches them, enabling subsequent fetches to iterate over the stored results without communicating with MySQL. This approach, known as "buffered queries," frees the MySQL server from further processing once the initial results are retrieved.

Conversely, unbuffered queries do not cache results, requiring the client to fetch them one at a time. This constraint arises when a second query is executed before the results of the first query have been fully fetched. The MySQL server interprets this as a conflict between active queries and triggers the error message.

Resolution:

To resolve this issue, the following strategies can be employed:

• Use fetchAll(): By invoking the fetchAll() method, all results of a query are retrieved and cached in PHP. This approach effectively simulates a buffered query, allowing subsequent queries to execute without running into the error.
• Enable PDO::MYSQL_ATTR_USE_BUFFERED_QUERY: Setting this PDO attribute to true forces the use of buffered queries, irrespective of the PDO::ATTR_EMULATE_PREPARES setting. This method ensures that all results are fetched implicitly upon the first fetch, preventing conflicts with subsequent queries.
• Close your cursors: Calling closeCursor() on a query explicitly notifies the MySQL server that its results can be discarded. This allows for the execution of new queries without encountering the error. It should be noted that closeCursor() should only be employed after all the intended results have been fetched.

Additional Considerations:

• The issue may arise intermittently due to alternating PHP version as compatibility with MySQL client libraries can vary.
• The error most commonly surfaces when there are multiple active queries, with each query involving a result set of more than one row.
• Switching to MySQL Native Driver (mysqlnd) is recommended, as it offers greater functionality and memory efficiency.

Code Example:

The provided code can be modified to prevent the error by utilizing fetchAll() or explicitly setting PDO::MYSQL_ATTR_USE_BUFFERED_QUERY to true.

$sql ='SELECT temp_id FROM temp1';
$stmt2 = db::db()->prepare($sql);
$stmt2->execute();
$rs2 = $stmt2->fetchAll(PDO::FETCH_ASSOC);
$stmt2->closeCursor();

$sql='SELECT COUNT(*) AS valid FROM cities_has_zipcodes 
      WHERE cities_id=:cities_id AND zipcodes_id=:zipcodes_id';
$stmt1 = db::db()->prepare($sql);

foreach($data AS $row)
{
    try
    {
        $stmt1->execute($row);
        $rs1 = $stmt1->fetchAll(PDO::FETCH_ASSOC);
        $stmt1->closeCursor();
        syslog(LOG_INFO,'$rs1: '.print_r($rs1[0],1).' '.rand());
        syslog(LOG_INFO,'$rs2: '.print_r($rs2[0],1).' '.rand());
    }
    catch(PDOException $e){echo(sql_error($e));}            
}

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