RE: "direct path read" and "db file sequential read" used for full table scans in 11g
Date: Fri, 31 Aug 2012 05:43:46 -0700
Message-ID: <-8651461411394617575_at_unknownmsgid>
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Bill"shrek" thater Oracle DBA
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From: Gaja Krishna Vaidyanatha
Sent: 8/31/2012 2:13 AM
To: Oracle-L List
Subject: Re: "direct path read" and "db file sequential read" used for
full table scans in 11g
Hi Allen,
Regardless of the new feature in 11g, I have observed "db file
sequential read" events on even simple full table scans like - select
/*+ FULL(t) */ count(*) from xxx t. This occurs when Oracle is unable
to read DBFMBRC worth of blocks from a given extent. This usually
happens towards the edge of the high-water-mark of the table, but in
theory can happen in any extent of the table.
For example, assume a given table has 8 extents (where each full extent is 16 blocks of real data) and also assume that the total number of blocks below the high-water-mark is 124. In this scenario, the trace file will show 7 calls of "db file scattered read" (direct path read in your case) and this accounts for 112 blocks. This will be followed by 12 calls of "db file sequential read" to account for the remaining blocks below the high-water-mark. So bottom line, even in full-table-scans or index fast-full scans, there could be occurrences of "db file sequential read" for Oracle to perform single block reads.
On a related note on setting DBFMBRC, I hope you are aware that from 10gR2 and up, the recommendation is to have Oracle set DBFMBRC automatically to X (where DB_BLOCK_SIZE * X = 1MB). This will assist in fewer read system calls and thus better elapsed times for full-scans, when compared to a smaller DBFMBRC set manually. This "automatic feature" (you basically comment out the parameter OR just remove it from the init.ora or spfile), allows the instance to have large I/O chunk-sizes and prevents overly influencing the Optimizer, that full-scans are cheap. So in a way, we get to have the cake and eat it too. Hope this helps.
Cheers,
Gaja
Gaja Krishna Vaidyanatha,
CEO & Founder, DBPerfMan LLC
http://www.dbperfman.com
http://www.dbcloudman.com
Phone - +1-650-743-6060
LinkedIn - http://www.linkedin.com/in/gajakrishnavaidyanatha
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Enabling Cloud Deployment & Management for Oracle & Big Data
From: "Allen, Brandon" <Brandon.Allen_at_OneNeck.com> To: "oracle-l_at_freelists.org" <oracle-l_at_freelists.org> Sent: Thursday, August 30, 2012 2:47 PM
Subject: RE: "direct path read" and "db file sequential read" used for full table scans in 11g
Question #3 is open again
According to the docs the answer I thought I'd found would only
explain what I'm seeing if I had the parameter
parallel_degree_policy=AUTO, but I just checked and it is set to
MANUAL, so the parallel queries should still be using direct path
reads according to the doc:
http://docs.oracle.com/cd/E11882_01/server.112/e25513/initparams179.htm#REFRN10310
SQL> show parameter PARALLEL_DEGREE_POLICY
NAME TYPE VALUE
------------------------------------ ----------- ------
parallel_degree_policy string MANUAL
From: Allen, Brandon
I found the answer to question #3 - apparently it's a new feature called "In-Memory Parallel Execution" as documented at the links below. I'm still looking for answers on #1 and 2 though if anyone has any ideas.
From: Allen, Brandon
1) How can I force production to use the direct path reads?
2) Why is it using "db file sequential read" for a full table scan instead of "db file scattered read"?
3) I also tried setting PARALLEL to 2 on the table in question and it did force Oracle to use a parallel execution plan, but it still used "db file sequential read" instead of "direct path read" - any idea why this could be?
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Received on Fri Aug 31 2012 - 07:43:46 CDT