Performance of One Query on fetch_musername Hook

[James Birkett]

I have one query being ran on the fetch_musername hook to grab the value of one custom user field. The query (off the top of my head) is:

$field = $vbulletin->db->query_first(sprintf("SELECT field16 FROM " . TABLE_PREFIX ."userfield WHERE userid = %d", $user['userid']));

I'm worried about the performance of this query and how it'll affect my server load.

Will this query be ran once and cached - or will it be ran every time the fetch_musername hook is ran?

If anyone can shed any light, that'd be great - thanks.