Relationship between a block of data and the latch

Hello Experts,

I just read an article on the full table scan and index by Richard Foote. http://richardfoote.WordPress.com/2009/04/16/indexes-on-small-tables-part-i-one-of-the-few/

I thought that each block required only a single lock for each operation. However, I don't undersand that, why a single block required 2 locks? The qoute is the following,

Note that Yes, indeed, he did 4 gets done and that none of the uniform becomes corresponded to the "cheapest" gets reviews. Therefore, the 4 in line gets used in the performance of the FTS of the required a single block table 4 x 2 = 8 snaps.

Can someone explain the above quote it please?

Thanks in advance.

in most of the cases the latch fails while the buffer is pinned - it is only necessary get pin code, then again to remove (although for very quick access operations, there is also the "get consistent - review" that holds the lock while accessing the buffer). Randolf Geist explained the reason of the PIN and unpin in related things Oracle: e/s logic - evolution: part 1 - Reference:

Why not simply Oracle retains all tampons pinned rather than go through the exercise of hash/latch/PIN again and again? Very probably for reasons of scalability / concurrency, for example:

-A pinned buffer can not be deleted/replaced even if she was eligible according to the logic of the LRU, so potentially prevent other caching buffers

-A pinned buffer is not accessible by other sessions that want to pin incompatible mode (exclusive vs. shared), although several sessions can pin simultaneously in compatible mode (shared). These two sessions are in the queue behind (that's what a "buffer busy wait" is on) or they may be able to create a clone of the block copy and to continue their work on copy of the clone.

Tags: Database

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