What exactly is the difference between enqueue,latch & lock

Question

Can someone explain in simple words (with example if possible) what exactly is the difference between enqueue,latch & lock?
I have gone through documentation & other links,but just not able to figure out the exact & clear difference between these three..
Cheers,
Kunwar

Answer 1

In addition to links provided by Sb and nice explanations by Aman, i would like to add below :
1.Latches and enqueues are lightweight serialization devices used to coordinate multi-user access to
shared data structures, objects and files.
Latches are locks that are held for extremely short periods of time, for example the time it takes to modify
an in-memory data structure. They are used to protect certain memory structures, such as the database
block buffer cache or the library cache in the shared pool (as described in Chapter 2, Architecture). Latches
are typically requested internally in a ‘willing to wait’ mode. This means that if the latch is not available,
the requesting session will sleep for a short period of time and retry the operation later. Other latches may
be requested in an ‘immediate’ mode, meaning that the process will go do something else rather than sit
and wait for the latch to become available. Since many requestors may be waiting for a latch at the same
time, you may see some processes waiting longer than others. Latches are assigned rather randomly, based
on the ‘luck of the draw’, if you will. Whichever session asks for a latch right after it was released will get it.
There is no line of latch waiters, just a ‘mob’ of waiters constantly retrying.
Oracle uses atomic instructions like ‘test and set’ for operating on latches. Since the instructions to set
and free latches are atomic, the operating system itself guarantees that only one process gets it. Since it
is only one instruction, it can be quite fast. Latches are held for short periods of time and provide a
mechanism for clean-up in case a latch holder ‘dies’ abnormally while holding it. This cleaning up
process would be performed by PMON.
Enqueues are another, more sophisticated, serialization device, used when updating rows in a database
table, fro example. They differ from latches in that they allow the requestor to ‘queue up’ and wait for
the resource. With a latch request, the requestor is told right away whether they got the latch or not.
With an enqueue, the requestor will be blocked until they actually attain it. As such, they are not as fast
as a latch can be, but they do provided functionality over and above that which a latch can offer.
Enqueues may be obtained at various levels, so you can have many ‘share’ locks and locks with various
degrees of ‘shareability’.
Source:Expert Oracle Database Architecture by Thomas Kyte
2.Good PPT at http://nocoug.org/download/2001-05/latches.ppt
3.Good PPT by Mark bobak at http://www.mi-oaug.org/Presentations/Understanding%20Locks%20and%20Enqueues.ppt
HTH
Girish Sharma