The LRU Algorithm and Full Table Scans

LRU算法与全表扫描

When the user process is performing a full table scan, it reads the blocks of the table into buffers and puts them on the LRU end (instead of the MRU end) of the LRU list.

当用户进程执行全表扫描时，它会把表中的块读到buffer中，并把这些buffer放到LRU列表的LRU端。

This is because a fully scanned table usually is needed only briefly, so the blocks should be moved out quickly to leave more frequently used blocks in the cache.

这是因为全表扫描通常只是被短时地使用，所以这些块会被很快的移除，腾出空间来给使用频率高的的block。

You can control this default behavior. of blocks involved in table scans on a table-by-table basis.

你可以逐表控制表扫描涉及到的块的这种默认行为。

To specify that blocks of the table are to be placed at the MRU end of the list during a full table scan, use the CACHE clause when creating or altering a table or cluster. You can specify this behavior. for small lookup tables or large static historical tables to avoid I/O on subsequent accesses of the table.

指定在全表扫描中将表中的块放在LRU列表的MRU端，可以在创建或是修改表或Cluter时使用CACHE从句。你可以为小的查询表或是大的静态历史数据表指定这种行为，以减少后续访问表时的I/O。

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LRU算法与全表扫描

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