RE: 9.2 V$ views
- From: Wolfson Larry - lwolfs <lawrence.wolfson@xxxxxxxxxx>
- To: "'oracle-l@xxxxxxxxxxxxx'" <oracle-l@xxxxxxxxxxxxx>
- Date: Mon, 26 Jul 2004 18:24:11 -0500
Tanel, thanks for the crystal clear explanation of the dba_extents problem.
Larry
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx]On Behalf Of Tanel P=F5der
Sent: Monday, July 26, 2004 6:13 PM
To: oracle-l@xxxxxxxxxxxxx
Subject: Re: 9.2 V$ views
> (like even analyzing DD in 9.2, etc.)
>
> Can't that lead us to other problems?
Yep, it can ;)
But in 9.2 it's supported and in 10g it's default anyway...
Btw, the dba_extents performance problem you mentioned earlier probably
comes from the fact that in LMT configuration you can get detailed extent
information only from the segment header itself, this means at least one
consistent get per segment which you're querying. Given that lots of segment
headers aren't in buffer cache, a count(*) on dba_extents may cause
thousands of physical IO's, recursive calls and latching...
SQL> set autot trace stat
SQL> select count(*) from dba_extents;
1 row selected.
Statistics
----------------------------------------------------------
41228 recursive calls
1 db block gets
20799 consistent gets
3389 physical reads
0 redo size
380 bytes sent via SQL*Net to client
511 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
So, dba_extents is a quite dangerous view to query in LMT environment.
Tanel.
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