Thursday, 7 November 2013

DBA_SEGMENTS Vs. DBA_EXTENTS for calculating data size

If you are dealing with a single datafile tablespace and you'd need to know the size of a specific chunk of data, you need to use DBA_SEGMENTS!

Here's a simple experiment on tablespaces with 32K Blocks, on ASM and 11.2.0.3, and PARALLEL_DEGREE_POLICY set to MANUAL:

SELECT /*+ PARALLEL(DE,4) */ SUM(BYTES)
FROM DBA_EXTENTS DE
WHERE TABLESPACE_NAME='ACTIX_DATAPART_0001';
-- Result: 8346468352 IN 162.67 SECs

SELECT /*+ PARALLEL(DS,4) */ SUM(BYTES)
FROM DBA_SEGMENTS DS
WHERE TABLESPACE_NAME='ACTIX_DATAPART_0001'
-- Result: 8346468352 IN 3.687 SECs
The SQL statements were run exactly in the same order that are displayed, but to make sure that the caching of the base segments (like SYS_OBJECTS view or any other SYS schema object) is not contributing to the achieved time, shared pool was flushed and SQL statements were run in reverse order. Same results.

The tablespace used has two datafiles, but same statements were also tried on single file tablespaces and again, the same results.

Obviously DBA_EXTENT would allow you to query based on FILE_ID, which -unfortunately- will be crucial when you are interested to know about segments/extents on one of the files. In my case, I need it for a "datafile defragmentation" proc, which requires the BLOCK_ID of extents; hence DBA_EXTENTS.

Side note: The SUM(BYTES) would return the total sum of the size of the blocks that contain data; not the actual size of the data, although I'm not sure whether that matters or not!

Post-publish-newbie-Observation: Just noticed that DBA_SEGMENTS is entirely based on SYS.SYS_DBA_SEGS, whose HEADER_FILE column represents FILE_ID.

Also that the "Bytes" column in DBA_EXTENTS/DBA_SEGMENTS is calculated using SYS_DBA_SEGS based in the following (ugly looking, but time saving!) formula:

decode(bitand(segment_flags, 131072), 131072, blocks,  (decode(bitand(segment_flags, 1), 1, dbms_space_admin.segment_number_blocks(tablespace_id,relative_fno, header_block, segment_type_id, buffer_pool_id,segment_flags,
segment_objd, blocks), blocks)))*blocksize

Regarding the EXTENT_ID: what you need is in sys.x$ktfbue.ktfbuebno; that is, if you need a precise BLOCK_ID, but if -like me, using a 5MB margin for resizing datafiles- a "more-or-less" precise BLOCK_ID does the job, then your answer lies in SYS.SYS_DBA_SEGS.HEADER_BLOCK.

It is either the same as the BLOCK_ID you'd get from sys.x$ktfbue.ktfbuebno and sys.uet$.block# (that is DBA_EXTENTS.BLOCK_ID) or it's only a mere 2 blocks higher!

The king is dead! All hail the king!

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