Question

I am trying to use a prepared statement that deletes from two (temporary) tables:

public function clearTempTables()  
{  
   static $delStmt = null;  
    if (null == $delStmt)  
    {  
        $delStmt = $this->pdo->prepare("DELETE FROM product_string_ids;   
                                        DELETE FROM product_dimension_value_ids;");  
    }  

    $delStmt->execute();
}

Calling this function succeeds, but when a statement is executed afterwards, I receive the following error:

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.

Setting PDO::MYSQL_ATTR_USE_BUFFERED_QUERY when creating my PDO object (as indicated in the docs and numerous web examples) seems to have no effect, and calling $delStmt->fetchAll() results in “SQLSTATE[HY000]: General error”, which makes sense, since delete statements shouldn’t return and results that need fetching anyway.

What am I doing wrong here? Is it even possible to prepare multiple SQL statements into a single statement like this? From a performance point of view it certainly makes sense, especially with a large number of queries that would do work on temporary tables and then only return a final result set.

Answer 1

Afaik it’s not possible to prepare multiple statements as one (combined) prepared statement.
But the DELETE syntax allows you to specify multiple tables to delete rows from.

$this->pdo->prepare("
  DELETE
    product_dimension_value_ids,product_string_ids
  FROM
    product_dimension_value_ids,product_string_ids
");

(now it’s tested)