Efficiently query DBA_EXTENTS for FILE_ID / BLOCK_ID

By Franck Pachot

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This was initially posted to CERN Database blog on Thursday, 27 September 2018 where it seems to be lost. Here is a copy thanks to web.archive.org

Did you ever try to query DBA_EXTENTS on a very large database with LMT tablespaces? I had to, in the past, in order to find which segment a corrupt block belonged to. The information about extent allocation is stored in the datafiles headers, visible though X$KTFBUE, and queries on it can be very expensive. In addition to that, the optimizer tends to start with the segments and get to this X$KTFBUE for each of them. At this time, I had quickly created a view on the internal dictionary tables, forcing to start by X$KTFBUE with materialized CTE, to replace DBA_EXTENTS. I published this on dba-village in 2006.

I recently wanted to know the segment/extend for a hot block, identified by its file_id and block_id on a 900TB database with 7000 datafiles and 90000 extents, so I went back to this old query and I got my result in 1 second. The idea is to be sure that we start with the file (X$KCCFE) and then get to the extent allocation (X$KTFBUE) before going to the segments:

So here is the query:

column owner format a6
column segment_type format a20
column segment_name format a15
column partition_name format a15
set linesize 200
set timing on time on echo on autotrace on stat
WITH
 l AS ( /* LMT extents indexed on ktfbuesegtsn,ktfbuesegfno,ktfbuesegbno */
  SELECT ktfbuesegtsn segtsn,ktfbuesegfno segrfn,ktfbuesegbno segbid, ktfbuefno extrfn,
         ktfbuebno fstbid,ktfbuebno + ktfbueblks - 1 lstbid,ktfbueblks extblks,ktfbueextno extno
  FROM sys.x$ktfbue
 ),
 d AS ( /* DMT extents ts#, segfile#, segblock# */
  SELECT ts# segtsn,segfile# segrfn,segblock# segbid, file# extrfn,
         block# fstbid,block# + length - 1 lstbid,length extblks, ext# extno
  FROM sys.uet$
 ),
 s AS ( /* segment information for the tablespace that contains afn file */
  SELECT /*+ materialized */
  f1.fenum afn,f1.ferfn rfn,s.ts# segtsn,s.FILE# segrfn,s.BLOCK# segbid ,s.TYPE# segtype,f2.fenum segafn,t.name tsname,blocksize
  FROM sys.seg$ s, sys.ts$ t, sys.x$kccfe f1,sys.x$kccfe f2 
  WHERE s.ts#=t.ts# AND t.ts#=f1.fetsn AND s.FILE#=f2.ferfn AND s.ts#=f2.fetsn
 ),
 m AS ( /* extent mapping for the tablespace that contains afn file */
SELECT /*+ use_nl(e) ordered */
 s.afn,s.segtsn,s.segrfn,s.segbid,extrfn,fstbid,lstbid,extblks,extno, segtype,s.rfn, tsname,blocksize
 FROM s,l e
 WHERE e.segtsn=s.segtsn AND e.segrfn=s.segrfn AND e.segbid=s.segbid
 UNION ALL
 SELECT /*+ use_nl(e) ordered */ 
 s.afn,s.segtsn,s.segrfn,s.segbid,extrfn,fstbid,lstbid,extblks,extno, segtype,s.rfn, tsname,blocksize
 FROM s,d e
  WHERE e.segtsn=s.segtsn AND e.segrfn=s.segrfn AND e.segbid=s.segbid
 UNION ALL
 SELECT /*+ use_nl(e) use_nl(t) ordered */
 f.fenum afn,null segtsn,null segrfn,null segbid,f.ferfn extrfn,e.ktfbfebno fstbid,e.ktfbfebno+e.ktfbfeblks-1 lstbid,e.ktfbfeblks extblks,null extno, null segtype,f.ferfn rfn,name tsname,blocksize
 FROM sys.x$kccfe f,sys.x$ktfbfe e,sys.ts$ t
 WHERE t.ts#=f.fetsn and e.ktfbfetsn=f.fetsn and e.ktfbfefno=f.ferfn
 UNION ALL
 SELECT /*+ use_nl(e) use_nl(t) ordered */
 f.fenum afn,null segtsn,null segrfn,null segbid,f.ferfn extrfn,e.block# fstbid,e.block#+e.length-1 lstbid,e.length extblks,null extno, null segtype,f.ferfn rfn,name tsname,blocksize
 FROM sys.x$kccfe f,sys.fet$ e,sys.ts$ t
 WHERE t.ts#=f.fetsn and e.ts#=f.fetsn and e.file#=f.ferfn
 ),
 o AS (
  SELECT s.tablespace_id segtsn,s.relative_fno segrfn,s.header_block   segbid,s.segment_type,s.owner,s.segment_name,s.partition_name
  FROM SYS_DBA_SEGS s
 ),
datafile_map as (
SELECT
 afn file_id,fstbid block_id,extblks blocks,nvl(segment_type,decode(segtype,null,'free space','type='||segtype)) segment_type,
 owner,segment_name,partition_name,extno extent_id,extblks*blocksize bytes,
 tsname tablespace_name,rfn relative_fno,m.segtsn,m.segrfn,m.segbid
 FROM m,o WHERE extrfn=rfn and m.segtsn=o.segtsn(+) AND m.segrfn=o.segrfn(+) AND m.segbid=o.segbid(+)
UNION ALL
SELECT
 file_id+(select to_number(value) from v$parameter WHERE name='db_files') file_id,
 1 block_id,blocks,'tempfile' segment_type,
 '' owner,file_name segment_name,'' partition_name,0 extent_id,bytes,
  tablespace_name,relative_fno,0 segtsn,0 segrfn,0 segbid
 FROM dba_temp_files
)
select * from datafile_map where file_id=5495 and 11970455 between block_id and block_id+blocks

And here is the result, with execution statistics:

   FILE_ID   BLOCK_ID     BLOCKS SEGMENT_TYPE         OWNER  SEGMENT_NAME    PARTITION_NAME    EXTENT_ID      BYTES TABLESPACE_NAME      RELATIVE_FNO     SEGTSN     SEGRFN    SEGBID
---------- ---------- ---------- -------------------- ------ --------------- ---------------- ---------- ---------- -------------------- ------------ ---------- ---------- ----------
      5495   11964544            8192 INDEX PARTITION LHCLOG DN_PK           PART_DN_20161022 1342         67108864 LOG_DATA_20161022            1024       6364       1024        162

Elapsed: 00:00:01.25

Statistics
----------------------------------------------------------
        103  recursive calls
       1071  db block gets
      21685  consistent gets
        782  physical reads
        840  redo size
       1548  bytes sent via SQL*Net to client
        520  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

Knowing the segment from the block address is important in performance tuning, when we get the file_id/block_id from wait event parameters. It is even more important when a block corrution is detected ans having a fast query may help.

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