Question

I have some questions about using MySQLi queries, and related memory management.

Suppose I have something like this:

$db = new mysqli($dbhost, $un, $ps, $dbname);

$query = "SELECT field1, field2  FROM table1 ";
$results = $db->query($query);

while ($result = $results->fetch_object()) {
    // Do something with the results
}

$query = "SELECT field1, field2 FROM table2 ";
// question 1
$results = $db->query($query);

while ($result = $results->fetch_object()) {
    // Do something with the second set of results
}

// Tidy up, question 2
if ($results) {
    $results->free();
}
if ($db) {
    $db->close();
}

// Question 3, a general one

So, based on the comments in the code above, here are my questions:

1. When I assign the results of the second query to $results, what happens to the memory associated with the previous results? Should I be freeing that result before assigning the new one?

2. Related to 1, when I do clean up at the end, is cleaning up just the last results enough?

3. When I do try to clean up a result, should I be freeing it as above, should I be closing it, or both?

I ask question 3 because the PHP documentation for mysqli::query has an example that uses close, even though close is not part of mysqli_result (see example 1 in the link above). And in contrast, my normal PHP reference text uses free (PHP and MySQL Web Development, Fourth Edition, Welling and Thomson).

Answer 1

When I assign the results of the
second query to $results, what happens
to the memory associated with the
previous results?

When you execute this:

$results = $db->query($query);

If there was something in $results before, this old content cannot be accessed anymore, as there is no reference left to it.

In such a case, PHP will mark the old content of the variable as "not needed anymore" — and it will be removed from memory when PHP needs some memory.

This, at least, is true for general PHP variables; in the case of results from an SQL query, though, some data may be kept in memory on the driver-level — over which PHP doesn’t have much control.

Should I be freeing that result before
assigning the new one?

I never do that.

Related to 1, when I do clean up at
the end, is cleaning up just the last
results enough?

When the scripts end:

• The connection to the database will be closed — which means any memory that might be used by the driver should be freed
• All variables used by the PHP script will be destroyed — which means the memory they were using should be freed.

So, at the end of the script, there is really no need to free the result set.

When I do try to clean up a result,
should I be freeing it as above,
should I be closing it, or both?

If you close the connection to the database (using mysqli::close like you proposed), this will disconnect you from the database.

This means you’ll have to re-connect if you want to do another query! Which is not good at all (takes some time, resources, … )

Generally speaking, I would not close the connection to the database until I am really sure that I won’t need it anymore — which means I would not disconnect before the end of the script.

And as "end of the script" means "the connection will be closed" even if you don’t specify it; I almost never close the connection myself.