Performance/Benchmark questions

[eluser]hykoh[/eluser]
I just have a few questions about the performance with CI

- What does the {memory_usage} calculate ? The RAM usage of the system or what else ?
- When should i optimize my system ? From which value of memory_usage / elapsed_time ?
At the moment my project takes 0.0602 / 1.31MB for one siteload. Is it too much ? it runs on a vserver without other load.

Another question is (i know - its maybe not the right forum, but nearly the same topic ), should i use

Code:

$sql = SELECT COUNT(*) AS cnt FROM example WHERE x = y
// i use PDO with a wrapper class
$stmt = $pdo->query($sql);
$row = $stmt->fetch();

return $row['cnt']

or

Code:

$sql = SELECT id FROM example WHERE x = y
// i use PDO with a wrapper class
$stmt = $pdo->query($sql);

return $stmt->numRows() // or known as mysql_num_rows()

Is it recommend to use COUNT about the database (if yes - should i use COUNT(id) or something like that instead of * ?)

Or should the num_rows() function get me more performance ?