Reclaiming space on Tables, Indexes,Lob Index,Lob Partition, Lob Segments


  There is a big database and we are having issues with space.     I want to reclaim unused space from  segments.

  a) What query can i perform that shows unused space of segments ?

  b)  I will like to know  how to reclaim unused space from these different type of segments :Tables, Indexes,Lob Index,Lob Partition, Lob Segments.

 c) How to reclaim space from LOB Columns.

joe_echavarriaDatabase AdministratorAsked:
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a select * from dba_free_space

b you can try to resize your data files, but if you have segments at the end of a file you can't shrink them beyond the last block of that segment

The usual way of reclaiming space is to move the objects at the end of a data file into a different tablespace.  Then shrink the datafile.  You can optionally move the objects back after the shrink and then drop the "holding" tablespace once it's empty, or just leave them there.  There's no technical advantage either way, it's more of an administrative convenience and your own internal standards.

c same as b
joe_echavarriaDatabase AdministratorAuthor Commented:
What about the below commands ?

ALTER TABLE  lob_test modify lob (data) (shrink space);


ALTER TABLE &table_name> LOB (&lob_column) (SHRINK SPACE;

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within an object,  yes, you can do that.

but.. that doesn't make your tablespace any smaller, so your database will still be just as big as it was before hand.

To use an analogy, if you have a 1 gallon bucket full of water and then you pour some of the water out, the water had "shrunk" but the bucket is still the same size.

But continuing the analogy - you now have more space in the bucket to add more water.
Same with a tablespace, if you shrink the objects within it, then you can grow those objects again, or add more objects.  The tablespace is still the same size.
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joe_echavarriaDatabase AdministratorAuthor Commented:
But having the less data in the tablespace might help us to improve some performance ?
johnsoneSenior Oracle DBACommented:
Shrinking the free space of objects isn't going to improve performance.

The only case where that would help is if you are doing full table scans and have lots of deletes and there is a large amount of space below the high water mark that is being scanned.
It "can" help, but usually not by much.

If you have  1000 rows of data spread across 1000 data blocks, then you'll need to read 1000 data blocks to get your data (full table scan or index read, maybe even more with full table scan)

If you shrink your data down to fewer blocks (i.e. more rows per block) then it requires fewer reads.

Full table scans as mentioned above are a particularly bad case because you'll read through empty blocks en route to finding data blocks.  

Index scans are at least guaranteed to point to "some" data", but you still might end up reading more blocks than necessary if those blocks have lots of free space in them.
DavidSenior Oracle Database AdministratorCommented:
I've been thinking about your original comment, "issues of space".  While the advice provided is from the best, I wonder if this is addressing your performance concern.  

For example, what evidence has led you to deduce there's a problem.  This could be something like your file systems are full; or there is a rise in I/O wait time; or there are years upon years of obsolete data; or the users perceive that interactive queries are taking longer than usual.

So please say more, if you can .
joe_echavarriaDatabase AdministratorAuthor Commented:
I have shrinking space and performance have improved a lot.
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