Are prepared statements cached server-side across multiple page loads with PHP?

Question

I learnt about prepared statements when making a JDBC-enabled Java application, and my app uses a connection pooling layer that assures me that prepared statements are cache

Answer 1

You're confusing what is happening at the PHP/Java tier with what's happenning in the database.

Yes, using prepared statements (usually) means that the execution plan is cached by the database itself (NOT the PHP/Java tier). However it does not follow that this always results in better performance - and an explanation of this would take several hundred pages. However I infer from what you've said elsewhere you are using MySQL as the DBMS which makes the discussion somewhat simpler (IIRC none of the storage engines implement histograms). Typically MySQL will be able to cache enough information about a schema to be able to generate a plan without any disk I/O. OTOH, using prepared statements mean a minimum of three round trips to the DBMS for each query (present statement, present params, retrieve results) while using inlined values eliminates on of these round trips. In the absence of histogram indexes, the value of the variables is irrelevant to the optimal plan detectable by the optimizer.

The fact that you are using PHP, or PHP-FPM or Java with single or persistent or pooled connections is irrelevant to whether or not prepared-statements are cached/re-used by the DBMS.

HTH

C.