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

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The LRU/TCH Algorithm

So we have linked lists of buffer headers, and each buffer header points to a buffer, and each buffer holds a copy of a block. What we need now is an algorithm for deciding which buffer to over-write if we need to read a new block into memory. The algorithm that Oracle uses is a modified LRU (least recently used) with touch count (TCH).

images Note In the following pages, I really ought to talk about things like “. . . the block held by the buffer pointed to by the buffer header that . . .” but this would make the wording very clumsy. A common practice, and one I shall adopt, is to say “the buffer” when you really mean “the buffer ...

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