Return-Path: Received: from air189.startdedicated.com (root@localhost) by orafaq.com (8.11.6/8.11.6) with ESMTP id hBIBgjD29325 for ; Thu, 18 Dec 2003 05:42:45 -0600 X-ClientAddr: 66.27.56.212 Received: from www3.fatcity.com (rrcs-west-66-27-56-212.biz.rr.com [66.27.56.212]) by air189.startdedicated.com (8.11.6/8.11.6) with ESMTP id hBIBgjo29320 for ; Thu, 18 Dec 2003 05:42:45 -0600 Received: (from root@localhost) by www3.fatcity.com (8.11.6/8.11.6) id hBIAa8K22330 for oracle-l@orafaq.com; Thu, 18 Dec 2003 02:36:08 -0800 Received: by fatcity.com (05-Jun-2003/v1.0g-b73/bab) via fatcity.com id 005DA4F9; Thu, 18 Dec 2003 02:39:25 -0800 Message-ID: Date: Thu, 18 Dec 2003 02:39:25 -0800 To: Multiple recipients of list ORACLE-L X-Comment: Oracle RDBMS Community Forum X-Sender: "Sultan Syed" Sender: ml-errors@fatcity.com Reply-To: ORACLE-L@fatcity.com Errors-To: ML-ERRORS@fatcity.com From: "Sultan Syed" Subject: Re: db block gets /consistent gets Organization: Fat City Network Services, San Diego, California X-ListServer: v1.0g, build 73; ListGuru (c) 1996-2003 Bruce A. Bergman Precedence: bulk Mime-Version: 1.0 Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: 7bit Mark I know you from the metalink. Thank you for your detailed explanation. Syed ----- Original Message ----- To: "Multiple recipients of list ORACLE-L" Sent: Thursday, December 18, 2003 11:24 AM > Syed, > > Oracle accesses blocks in one of two modes, current or consistent. > > A 'db block get' is a current mode get. That is, it's the most up-to-date > copy of the data in that block, as it is right now, or currently. There > can only be one current copy of a block in the buffer cache at any time. > Db block gets generally are used when DML changes data in the database. > In that case, row-level locks are implicitly taken on the updated rows. > There is also at least one well-known case where a select statement does > a db block get, and does not take a lock. That is, when it does a full > table scan or fast full index scan, Oracle will read the segment header > in current mode (multiple times, the number varies based on Oracle version). > > A 'consistent get' is when Oracle gets the data in a block which is consistent > with a given point in time, or SCN. The consistent get is at the heart of > Oracle's read consistency mechanism. When blocks are fetched in order to > satisfy a query result set, they are fetched in consistent mode. If no > block in the buffer cache is consistent to the correct point in time, Oracle > will (attempt to) reconstruct that block using the information in the rollback > segments. If it fails to do so, that's when a query errors out with the > much dreaded, much feared, and much misunderstood ORA-1555 "snapshot too old". > > As to latching, and how it relates, well, consider that the block buffers > are in the SGA, which is shared memory. To avoid corruption, latches are > used to serialize access to many linked lists and data structures that point > to the buffers as well as the buffers themselves. It is safe to say that > each consistent get introduces serialization to the system, and by tuning > SQL to use more efficient access paths, you can get the same answer to the > same query but do less consistent gets. This not only consumes less CPU, > it also can significantly reduce latching which reduces serialization and > makes your system more scalable. > > Well, that turned out longer than I planned. If you're still reading, > I hope it helped! > > -Mark > > -----Original Message----- > From: Sultan Syed [mailto:ssyed@fine.ae] > Sent: Thu 12/18/2003 1:39 AM > To: Multiple recipients of list ORACLE-L > Cc: > Subject: db block gets /consistent gets > Hi list, > > What is db block gets and consistent gets.? > How can I reduce consistent gets ? > Ask Tom says each consistent gets is latch, how it could be? > Thanks in advance > > Syed > > > > > > -- > Please see the official ORACLE-L FAQ: http://www.orafaq.net > -- > Author: Bobak, Mark > INET: Mark.Bobak@il.proquest.com > > Fat City Network Services -- 858-538-5051 http://www.fatcity.com > San Diego, California -- Mailing list and web hosting services > --------------------------------------------------------------------- > To REMOVE yourself from this mailing list, send an E-Mail message > to: ListGuru@fatcity.com (note EXACT spelling of 'ListGuru') and in > the message BODY, include a line containing: UNSUB ORACLE-L > (or the name of mailing list you want to be removed from). You may > also send the HELP command for other information (like subscribing). -- Please see the official ORACLE-L FAQ: http://www.orafaq.net -- Author: Sultan Syed INET: ssyed@fine.ae Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru@fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).