Should I use prepared statements for MySQL in PHP PERFORMANCE-WISE?
I understand the security benefits of prepared statements in MySQL. No need to cover that topic here. I'm wondering about the performance aspect of them.
Now, I know when a query using a prepared statement is executed twice in a single PHP script, it's faster because the query is only parsed once, once for each query. The client makes one trip to prepare, then sends data twice using the binary protocol. The binary protocol is faster, and you're not taking the hit of having to parse a second time.
However, what about the case where I only want to perform a query once in a single PHP script? It would seem using a prepared statement is worse, because you're making two trips to the server, once to prepare, and once to send the data. The benefit of only having to parse once is lost, and you're penalized for that second trip. If the data isn't sufficiently smaller in binary format, you lose by using a prepared statement, right?
However, I've read some conflicting reports about what PHP's mysqli or PDO libraries do? Do either of them cache the prepared statement across script execution? Is the server going to have to parse the prepared statement again on a subsequent pageload or not? If the answer is no, that the statement doesn't have to be parsed on the second pageload, then it would seem that prepared statements ARE better, even if you're only executing the query once per pageload.
Please take into consideration if anything has changed between versions of MySQL regarding this. You can safely assume I'm using PHP 5.2
EDIT: Just to make it clear, I want an answer for MySQL and PHP specifically, specifying the MySQL version and if this was ever different, and to ONLY consider performance, not ease of use or security.
UPDATE: I accepted the answer I did because of the follow up comment had a few good ideas. I'm still a bit disappointed that no one seems to be able to answer the crux of the actual question I asked with any certainty. I guess sometimes the answer really is "it depends."
Answer
The History
This was my first Stackoverflow answer. A lot has changed since, specially the deprecation and removal of the mysql API. Even if you are still on php 5.6, the mysql_* api should not be used. Now PDO or mysqli are the only options to choose. PDO is better to lots of reasons.
Are prepared statements cached across page loads?
I've read some conflicting reports about what PHP's mysqli or PDO libraries do? Do either of them cache the prepared statement across script execution?
The same prepared statement will not be used in between page loads. It has to be prepared every time. If squeezing every large millisecond matters, a stored procedure might be a good idea (assuming you have a complicated query).
For large inserts (thousands of rows) A bigger boost can probably be gained by dumping your data into a text file and loading it with LOAD DATA IN FILE . It's a lot faster than a series of inserts.
The original answer
The truth of the matter is that sometimes mysqli is faster and at other times mysql api is faster. But the difference is really really small. If you look at any of the performance tests on the web the difference is really just 10 - 20 milliseconds. The best way to boost performance is to optimize table design.
Many of the tests that 'prove' the older api to be faster conveniently forget that for maximum security mysql_real_escape_string() should be called for each variable used in the query.
Queries are cached by the server, if and only if the data on all the tables that are used in the query have remained unchanged.
Await another update with actual numbers