Mysql fetch() returns FALSE - php

Question

I have currently a problem on streaming results from a mysql table. This table has more than 10,000,000 rows. And the structure
ID, IndexField1, Data1
With Primaryindex on ID and Index on IndexField1.
In PHP I tied to stream parts of the data (about 7 Mio) to calculate and verify some parts. For this reason I hava a callback which is been called for every single response.
public function streamForIndexedColumn(int $columnId, callable $callback){
$stmt = NULL;
$sql = "SELECT id, indexedField, data1 FROm myTable WHERE indexedField1 = ?;";
if (! ($stmt = $this->mysqlConn->prepare ( $sql ))) {
throw new Exception("ERROR");
}
try {
if (! $stmt->bind_param ( "i", $columnId)) {
throw new Exception( "Binding parameters failed: (" . $stmt->errno . ") " . $stmt->error );
}
$item = new Item ();
if (!$stmt->bind_result($item->id,$item->indexfield1,$item->data1)) {
throw new Exception( "Binding output parameters failed: (" . $stmt->errno . ") " . $stmt->error );
}
if (! $stmt->execute ()) {
throw new Exception("Execute failed: (" . $stmt->errno . ") " . $stmt->error );
}
$cnt = 0;
while ( $row = $stmt->fetch() ) {
$cnt++;
$callback($cnt, $item);
}
if ($row ===FALSE){
throw new Exception("MySQL Error: (" . $stmt->errno . ") " . $stmt->error );
} finally{
$stmt->close ();
}
}
This runs sometimes for several hours. Sometimes it is running, sometimes it is failing with message "MySQL Error: (0) "
Are I am getting the error reason in a wrong way? Are there any configurable Limits on the server or client which may produce this error?
I am using Mysql 5.17.19 and PHP 7.0.29.

Answer 1

The exception in $row === FALSE will always be thrown because after it fetches all rows, $row will be false.
$stmt->errno will return 0 because (Quoted from the docs: http://php.net/manual/en/mysqli.errno.php)
Returns the error number from the last MySQL function, or 0 (zero) if no error occurred.