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SQL> select count(*) from users;
Statistics
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0 recursive calls
2 db block gets
10 consistent gets
0 physical reads
db block gets 和consistent gets 两者有什么区别。
各位辛苦点给我简单介绍一下。我自己找了些英文资料。看了tom的回答,还是有点犯糊涂;
db block gets :
当前模式下,只读取现在存在的数据块。这种方式并不象在“读一致性样式”那样。正常情况下,当一个查询开始时候,符合该查询条件的块如果存在,就被获得,读取。Current Mode只要那些块现在(不是从在此之前的某个时间点算起)就存在那儿,就读取
(Current mode blocks are retrieved as they exist right now, not in a consistent read fashion.Normally, blocks retrieved for a query are retrieved as they existed when the query began. Current mode blocks are retrieved as they exist right now, not from a previous point in time.During a SELECT, you might see current mode retrievals due to reading the data dictionary to find the extent information for a table to do a full scan (because you need the "right now" information, not the consistent read). During a modification, you will access the blocks in current mode in order to write to them. )
consistent gets: 这是指在“读一致性样式”下,你总共处理的数据块数目.包括为了回滚而被读入回滚段的数据块,当你做一些编辑、删除操作时,首先在“读一致性样式”你会读取数据块,然后在”当前模式“下进行真正的编辑工作
偶英文差了点,翻译的很费劲,很难理解。
还请高手名师,指点。 |
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