When there are lot updates, deletes inside database, it creates lot of empty pockets of space that are not large enough to insert new data. We call this type of empty space as fragmented free space. Database performance can be impacted by such fragmented space. The process of combining fragmented space into one big free space is known as de-fragmentation. One of the simplest ways to do it by shrinking table, index segments to reclaim the wasted space. But before you can directly shrink table / index, you must run Oracle segment advisor to get recommendations as to how much space can you reclaim. In the below activity I will show you how to work with Oracle segment advisor: Full article link is here: https://www.support.dbagenesis.com/post/oracle-segment-advisor

1. ORACLE
SEGMENT
ADVISOR

2. Create test tablespace
We will create a separate tablespace to store new table for this
activity

3. Create test table & index
Let us create a test table, insert some records and create an index into the
above tablespace
Note: you can use this method to create new tables into a different
tablespace even though the default tablespace of a user is different.

4. Insert some dummy 100K records inside the table

5. Create index on table

6. Analyze table & index
Gather table and index stats

7. Let us check the size of each segment (table and index)

8. Delete some rows
Let us delete half of the rows from the table. Note, the below
procedure is deleting alternate rows from the table which will create
lot of fragmented space

11. Analyze table & index again
Now that we have delete half of the rows, let us gather stats
for table and index again
Let us check the size of each segment (table and index)
Note: even after deleting half of the records, the segment size
will still show same as it was before delete.

13. Run Oracle segment advisor
Even though we have delete half of the rows, the statistics
show no change. We will run Oracle segment advisor and see
what recommendations we will get
Create advisor task for table

16. Verify tasks are completed

17. Review segment advisor
Let us check what Oracle segment advisor has to say

18. Mostly the advisor will recommend you to run shrink space
command on the table and give an estimate as to how much
space can be saved.

19. Shrink table cascade
Use below command to shrink table but before that you must
enable row movement

20. Note: the CASCADE option will shrink space for associated
indexes with table. No need to run separate command for
index.

21. Analyze table & index size
Post shrinking table and index, let us gather stats and check
segment space savings

22. Let us check the size of each segment (table and index)
Note: this time the segment space size has been reduced from 797
earlier to 393.

23. Deallocate space
Now that we have shrink table and index, we can release the
deallocated space to the tablespace so that new data can be
inserted

24. At this stage, you can delete the advisor tasks that you
created earlier

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