As long as we're discussing major database overhauls, my main concern is moving to proper use of referential integrity and transactions. Locking entire tables has a very detrimental effect on high-traffic site performance. It also messes with existing transactions. Of course, this would require dropping MyISAM support. Transactions need not be annoying. I very recently released a module to make transactions much easier: http://drupal.org/project/pressflow_transaction Yes, it requires PHP 5 to perform its magic, but so does PDO. Larry Garfield wrote:
This is an RFC of sorts. :-)
At Drupalcon, I was shopping around the idea of a PDO[1] backend for Drupal. For brevity, I'll just reference the blog entry I made on why I think PDO is a good idea[2]. The goal is that PHP 5 users get a nice speed bump and we get a start on eventually shifting Drupal's database layer from the 1997 APIs to the more robust 21st century PHP standards. Most people I talked to liked the idea, so after getting a nod from Dries I've been working on it since the Hackfest and made decent progress. However, I've run into a few snags I want to get a 2nd opinion on (or 3rd, or 4th, or whatever).
1) It turns out that PDO from PECL running under PHP 5.1.6, at least, has issues. Specifically, segfault issues on otherwise perfectly sane queries. This is apparently a known issue with 5.1.6, and one of the reasons 5.2 exists. :-) At present, I have no solution for it other than saying "well, the PDO support is only if you're running 5.2, otherwise use the existing MySQL/PostgreSQL drivers, deal". Does anyone have a problem with that, and if so, an alternate solution?
2) Like any wrapper, PDO, while it offers some really nice features with a common API (like C-level prepared statements, which are what I'm mainly after), has some "lowest common denominator" issues. The main one I've run into so far is that there is no reliable equivalent of mysql_num_rows() for SELECT statements, only for data-changing statements[3]. In testing it doesn't look like the MySQL PDO driver returns anything useful for rowCount() on SELECT. That gives us 3 options.
A) Stop using db_num_rows() on result sets. (It's a database-specific feature in the first place.) In places where we use it, use a separate count(*) query instead. That's the more database-agnostic method, and is what the PHP manual recommends[3]. If we go this route, it will involve removing the db_num_rows() function from the existing mysql and postgres drivers and refactoring core queries accordingly.
B) Instead, have the PDO wrapper do a full ->fetchAll() on the result set. That gives an array of array or object records (specified in the fetchAll()), which can then be simply sizeof()ed in the PDO/db_num_rows() implementation. The downside here is that you need to specify in the fetchAll() whether you want objects or arrays. To not horribly break any existing APIs (I'm trying to minimize the footprint for now; we can break everything later), we'd need to therefore fetchAll() as an array in the PDO driver and then cast back to an object in db_fetch_object() (or vice versa). That feels quite nasty to me, honestly, and is probably a not-unnoticeable performance hit.
C) Drop the PDO idea. It should come as no surprise that this is my least favorite option. :-(
As my goal is that in some mythical future when we can require PHP 5 PDO becomes our main database backend (and we can leverage it even more for a cleaner API, faster/more stable prepared statements, etc.), I would favor slimming down and generalizing our database calls now (option A) so that we're better prepared in the future. (It would also make adding support for other database engines like Oracle or MS SQL easier.) I am, however, open to input and suggestions.
I am hoping I can get this into Drupal 6, but time will tell if I can get it ready in time.
[1] http://www.php.net/pdo [2] http://www.garfieldtech.com/blog/drupalcon-php5 [3] http://us.php.net/manual/en/function.PDOStatement-rowCount.php