I figure it's "a must" to blog any time I learn
something new. I've set events on
databases many times over the years but for the most part I've never paid that
much attention to the syntax. I knew
about SESSION vs. SYSTEM, that CONTEXT lets you define how long the event
should be active, etc., but I never paid attention to the impact of EVENT vs.
EVENTS.
Using "ALTER SYSTEM SET EVENT '…';" changes the parameter
EVENT and overrides previous settings (more on that later), "ALTER SYSTEM SET EVENTS '…';" makes the listed event active
immediately but not persistent across database restarts. I guess that should be obvious, because
without specifying SCOPE the change would only be in memory. What makes this different, though, is that
using EVENT gives you the ability to define SCOPE (and SID) while EVENTS does not. Using
EVENTS actually changes a memory structure (see Tanel Poder's blog on
"Why doesn't ALTER SYSTEM SET EVENTS set the events or
tracing immediately?": http://tinyurl.com/kh852ea).
Back to EVENT, setting it to a value overrides previous
setting(s), just as it would for any other instance/database parameter. This means it's important to first check for
any existing values for EVENT before setting new one(s).
