How do odbc (or mysql) resources work in php?

Question

When you run a query like so:

$query = "SELECT * FROM table";
$result = odbc_exec($dbh, $query);
while ($row = odbc_fetch_array($result)) {
    print_r($row);
}

Does the resource stored in $result point to data that exists on the server running php? Or is pointing to data in the database? Put another way, as the while loop does it's thing ,is PHP talking to the DB every iteration or is it pulling that $row from some source on the application side?

Where this is mattering to me is I have a database I'm talking to over VPN using ODBC with PHP. This last weekend something strange has happened where huge pauses are happening during the while loop. So between iterations, the script will stop execution for seconds and up to minutes. It seems to be completely random where this happens. I'm wondering if I need to talk to the server over VPN each iteration and maybe the connection is flaky or if something has gone wrong with my ODBC driver (FreeTDS).

Answer 1

mysql_query and odbc_exec both return a resource which (quote from php.net) "is a special variable, holding a reference to an external resource." This suggests the server is talking with the database server every iteration, I am not sure though.

However, there are 2 connections we are talking about here. The first being your connection with the PHP server, and the second one being the connection between the PHP server and the database server. If both servers have a fast connection, the strange behaviour you are experiencing might not have anything to do with your VPN.

Answer 2

The resource identifies the internal data structure used by PHP for interacting with the external resource.

In the case of the resource returned by mysql_query(), this data structure will include the rows returned by the query (and won't return until all the data has been returned or the conenction fails). However this behaviour is specific to MySQL - there is no requirement that the DBMS return the data before it is explicitly requested by the client.

If there is some strange problem causing lots of latency in your setup, then the only obvious solution would be to compile the results of the query at the database side then deliver them to your PHP code, aither batched or as a whole (think webservice).

C.