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Oracle中幾種讀的關(guān)系 時間：2015/6/28來源：IT貓撲網(wǎng)作者：網(wǎng)管聯(lián)盟我要評論(0)

　　recursive calls: oracle自己偷偷執(zhí)行的，不去關(guān)注

　　db block gets: current read,直接讀buffer

　　consistent gets: consistent read，讀rollback segment中Undo后讀

　　physical reads:read from disk

　　因為logical reads=current reads+consistent reads=physical reads+ non-phisical reads

　　所以hit ratio= 1-physical reads/logical reads=1- physical reads(consistent gets+current reads)

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　　以下是網(wǎng)上摘來的文檔，注意紅色的那段我認為是描述不正確的，current read不是相對buffer而言，二是相對consistent read而言

　　· Recursive Calls. Number of recursive calls generated at both the user and system level.

　　Oracle Database maintains tables used for internal processing. When it needs to change these tables, Oracle Database generates an internal SQL statement, which in turn generates a recursive call.

　　In short, recursive calls are basically SQL performed on behalf of your SQL. So, if you had to parse the query, for example, you might have had to run some other queries to get data dictionary information. These would be recursive calls. Space management, security checks, calling PL/SQL from SQL—all incur recursive SQL calls.

　　· DB Block Gets. Number of times a CURRENT block was requested.

　　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.

　　(DB Block Gets:請求的數(shù)據(jù)塊在buffer能滿足的個數(shù))

　　· Consistent Gets. Number of times a consistent read was requested for a block.

　　This is how many blocks you processed in "consistent read" mode. This will include counts of blocks read from the rollback segment in order to roll back a block.

　　This is the mode you read blocks in with a SELECT, for example.

　　Also, when you do a searched UPDATE/DELETE, you read the blocks in consistent read mode and then get the block in current mode to actually do the modification.

　　(Consistent Gets:數(shù)據(jù)請求總數(shù)在回滾段Buffer中)

　　· Physical Reads. Total number of data blocks read from disk. This number equals the value of "physical reads direct" plus all reads into buffer cache. (Physical Reads:實例啟動后，從磁盤讀到Buffer Cache數(shù)據(jù)塊數(shù)量)

　　· Sorts (disk). Number of sort operations that required at least one disk write. Sorts that require I/O to disk are quite resource intensive. Try increasing the size of the initialization parameter SORT_AREA_SIZE.

關(guān)鍵詞標(biāo)簽：Oracle

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