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Oracle Core: Essential Internals for DBAs and Developers by Jonathan Lewis

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Latches

Latches come in two flavors—exclusive and shared read (the shared read latches became relatively common in 9i but, as I learned very recently, there were a few in Oracle 8.0), and to make things a little more confusing, it is possible to acquire a shared read latch in exclusive mode—so from now on I’ll refer to them as shareable latches.

There are also two ways of categorizing latch activity: the willing to wait gets (most of them) and the immediate gets. And again we have an area of overlap because some latch activity starts as immediate and then repeats as willing to wait, but we’ll ignore this topic for the moment.

Essentially a latch is the combination of a memory location in the SGA and an atomic CPU operation that can be used to ...

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