I would be grateful if you could tell me for what and in which processes the above function is used.
It is, on occasion, causing problems by putting the CPU on the webserver under heavy load (I am informed) by going through all the databases on the server.
Many thanks,
Richard

That is not a "function", it is the base database handler class which the actual selected db handler PDO or mysqli are child classes from. It is therefore used everywhere.

There is a change in 1.7a that removes one unnecessary query that had already been reported. I don't recommend to use 1.7a naturally and I cannot tell if you can use it directly, but you can try:
https://github.com/zenphoto/zenphoto/commit/feba6501e42a0080cd467deebf3db2f61ce9f6b5

Thank you - I will try it.