Errm... if I may - that's not a very good example of using prepare_cached(): just moving the prepare() out from the for loop and using placeholders would have worked fine (and yes, I'm fairly sure that you know this - just pointing this out for others reading this thread).
prepare_cached() is really for the situation where you are likely to call a particular query more than once, possibly from different parts of your program, but not sequentially. In that situation DBI and the RDBMS will keep a copy of the query (and its query plan) on hand and re-use it when it is requested. You should keep in mind when using it that prepare_cached() will consume resources on the database server, because it will keep all of the queries that each client requests on hand/in cache until the clients disconnect. In some cases these query plans can be shared between clients (i.e. two different clients executing the same query), but not always (in particular I don't think that Sybase and/or MS-SQL will share query plans for prepared queries that use placeholders).
These are all items that need to be kept in mind - as with most things that pertain to database tuning/optimization the advisability of using a particular solution "depends" on the local circumstances.
Michael
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