As far as aggregate functions go, yes. The UDFs covered in Chapter 2 will do this. But if you want to modify the underlying SQL language (by adding new keywords that aren't functions for example) then no, this cannot yet be done with plugins.

Not yet unfortunately, it would be quicker to edit the parts of the MySQL source than to try this via. a plugin right now. There is a book called Understanding MySQL Internals that could help with this if it something you wanted to look in to. I know that is something they want to eventually do at MySQL (and something we want to eventually do for Drizzle too), but it is much harder to implement than it sounds.

There is a third party plugin called PBMS which I think has what you are looking for http://www.blobstreaming.org/ has more information.

Ah, I see what you mean. Not yet, it would be a real gem if you could alter the lexical parser in a plugin but it is really difficult to do at the moment. The closest alternative would probably be mysql-proxy, in which you could write a LUA script to intercept that and do what you wanted.

Unfortunately no... Well, you have the standard private/protected/etc... for the C++ parts. But I suspect it would not be difficult to get around those. The basic API has basic pre-defined functions, but for the more complex types (such as storage engines) you need to include some aspects of the original MySQL source code when compiling. The original idea of the MySQL Plugin API was to make it easier for people who already (or intend to) hack in raw code to the MySQL source. So the plugin is dynamically linked in like any other library, and if it crashes then the MySQL daemon crashes. I suspect (without any firm evidence) that the kind of security you are talking about would have a significant performance trade-off. It should also be noted that (at the moment) the plugins need to be compiled specifically for your version of MySQL, so in most cases you will see the source first. There is the initial work for 'services' which will end the need for this in many cases in MySQL 5.5 and MariaDB and this is talked about in the Appendix. I hope that kind of answers your question there :)

'Feature' is a very broad definition. But if you are talking about functions you could use Stored Functions and/or UDFs. These would, however, need a different name for the function call. Andrew Hutchings (co-author)

At the moment you have two options. The first is the Full-Text parser plugins, but this probably won't help you if you are handling INTs and may not quite be flexible enough for what you are looking for. If this is what you are interested in you may want to look at how products like Sphinx search engine does it, as that has a MySQL plugin. The second is indeed the storage engine plugin API. You could theoretically take an existing engine (such as the InnoDB plugin) and modify it in plugin form to work however you choose. Andrew Hutchings (co-author)

Yes, this plugin type is in MariaDB and should be coming to MySQL soon. There are examples in the Appendix of the book. As a side note we already have authentication plugins in Drizzle (what I work on now). Andrew Hutchings (co-author)

Yes, as with many plugin APIs there is the possibility of poor or malicious code opening up an attack vector. Off the top of my head I suspect the biggest risk would be from say bad string process in the Full-Text parser plugins which crashes the server. I would expect most plugins to come from in-house development where there is a special need for a unique setup and this type of user would be hacking code on the server anyway with the same (or worse) security implications. Andrew Hutchings (co-author)