Re: How does it know?

Neil Truby wrote: > Someone asked me a question about nreads and nwrites in output from > onstat -u. I'd always taken the nreads to be the number of physical pages> required to be retrieved from disk in order to satisfy the query (ie, data > not available in a page already in the buffer cache). I still think this. > > The question was, how does the engine know at the time of starting the > thread whether to increment the nreads count? Presumably it can't, since it > doesn't know in advance whether the requisite page will be available in > cache. So when is the count incremented? Presumably, when the physical read occurs, there is a record of which session requested the data, and the session count is updated appropriately, along with the global (onstat -p) equivalent count. The database is supposed to know what it is doing, and for whom it is doing it. -- Yours, Jonathan Leffler (Jonathan.Leffler@Informix.com) #include <disclaimer.h> Guardian of DBD::Informix v1.00.PC1 -- http://www.perl.com/CPAN "I don't suffer from insanity; I enjoy every minute of it!"