fetchOne vs. fetchAll vs. Sql's limit

Dear list, As you already know, there are different ways to retrieve a record-set from a database. For example, here's the classic way to perform this in PHP: while ($row = mysql_fetch_assoc($db)) { // my row is available here } Speacking about PHP internals, what exactly happens here? 1) The PHP asks the Database-Layer to give a record in each while pass? I mean the record-set is not available in PHP's memory, but in MySQL's Mem. 2) Or the database-layer has gave the whole result-set to PHP, and we're just fetching a single row from PHP's memory? 3) Finally, what is the efficient way to retrieve thousands or records which need to be processed at once? Using LIMIT? Kind regards, -behzad