Hi, all,

Periodically we have space alert on sys.301.499 of the prod database. Aaron 
asked me to investigate whether we need to do anything about it. This 301.499 
is a CACHE type segment at file# 301 block# 499 (by the way, one other type of 
segment named by file#.block# is temporary segments). Metalink note 18926.997 
says:

********** begin quote **********
I have an automated check on segments that cannot extend, and in all my systems 
it picks up the cache segment: 

Segements that won't be able to extend due to max/min extents 

OWNER SEGNAME SEGTYPE EXTENTS MAX_EXTENTS MIN_EXTENTS MBYTES TABLESPACE_NAME 
------- -------------------- ------- --------- ----------- ----------- 
--------- ------------------------------ 
SYS 1.285 CACHE 1 0 0 .00390625 SYSTEM 

Another thread in the forum indicated that this segment is used for sorting. If 
so, can I increase the size for performance? And should I be concerned that it 
is only one extent, or that it is defined with max_extents of zero? 

--------------------------------------------------------------------------------

From: Oracle, COBY ADAMS 07-Nov-00 14:31 
Subject: Re : cache segment 

Steve, 

Do not do anything to this segment. 

Please refer to the note below: 

<Note:107709.1> 
[snipped]
When the database is created, a segment of type 'CACHE' is created. It is 
owned by 'SYS' and resides in the system tablespace. It can been found 
by querying DBA_SEGMENTS or USER_SEGMENTS for a segment_type of 'CACHE' 
and has one extent allocated but MAX_EXTENTS = 0.
[snipped]
********** end quote **********

I believe that answers the question. So we can safely ignore the space alert on 
it.

If you're still interested in this segment type, Steve Adams' Web site is 
probably the only reference we can find outside of Oracle's Web site:

http://www.ixora.com.au/q+a/dd.htm:

CACHE segment  22 July 1999  
I was just curious about the segment types in Oracle (7.3.4), and found that 
there is a CACHE segment called 1.377. What is this CACHE segment all about?  
  In the old days this segment was used to bootstrap the data dictionary cache 
- hence the name. However, that job is now done with BOOTSTRAP$, so the only 
thing left in the CACHE segment is the compatibility information. That is, the 
list of features in use in the database that might prevent the database being 
opened with an earlier release of Oracle. The name of the CACHE segment, as for 
temporary segments, is derived from the file and block numbers of its segment 
header block.

If even that does not satisfy your curiosity, I dumped block 499 at file 301 of 
pifa on tpn into the file tpn:/oracle/trace/pifa/udump/pifa_ora_10501.trc. The 
database pifa is a partial clone of prod on hera. It contains the same CACHE 
segment at the same location. The trace file indicates that the segment 
contains database compatibility information, as Steve Adams says.

Yong Huang

[20041015 update]
Last version of Oracle where I see cache segment type is 9.2. Data block dump
shows: type: 0x0d=Compatibility segment. This segment no longer exists in 10g.