Table Fragmentation - Oracle

Table fragmentation –

If a table is only subject to inserts, there will not be any fragmentation.
Fragmentation comes with when we update/delete data in table.

The space which gets freed up during non-insert DML operations is not immediately re-used (or sometimes, may not get reuse ever at all). This leaves behind holes in table which results in table fragmentation.

To understand it more clearly, we need to be clear on how oracle manages space for tables.
“High water mark” of table actually defines the border line between (ever) used and unused (never) space. While performing full table scan, Oracle will always read the data up to HWM. And if there is lot of free space with-in HWM, that is read too, and hence degrading the performance of FTS.

Now lets see how to identify HWM, unused (never used) space and free space (used but deleted/updated) and then take a call whether the concerned table is candidate for a reorganization or not.

SQL> create table test as select * from dba_tables; -- Create a table
Table created.

SQL> exec dbms_stats.gather_table_stats('SYS','TEST); -- Gather Stats on this table


SQL> select blocks "Ever Used", empty_blocks "Never Used", num_rows "Total rows"
2 from user_tables where table_name='TEST'; -- The number of blocks used/free

Ever Used Never Used Total rows
---------- ---------- ----------
49 6 1680

SQL> delete from test where owner='SYS'; --- Im deleting almost half the number of rows.
764 rows deleted.

SQL> commit;
Commit complete.

SQL> exec dbms_stats.gather_table_stats('SYS','TEST); -- Gather Stats on this table again


SQL> select blocks "Ever Used", empty_blocks "Never Used", num_rows "Total rows"
2 from user_tables where table_name='TEST'; -- No difference in blocks usage

Ever Used Never Used Total rows
---------- ---------- ----------
49 6 916


Even though you deleted almost half the rows, the above shows that table HWM is up to 49 blocks, and to perform any FTS, Oracle will go up to 49 blocks to search the data. If your application is so-written that there are many FTS on this table, you may consider, reorganizing this table.

Reasons to reorganization

a) Slower response time (from that table)
b) High number of chained (actually migrated) rows.
c) Table has grown many folds and the old space is not getting reused.
Note: Index based queries may not get that much benefited by reorg as compared to queries which does Full table scan.

How to reorganize?

Before Oracle 10g, there were mainly 2 ways to do the reorganization of the table

a) Export, drop, and import.
b) Alter table move (to another tablespace, or same tablespace).

Oracle 10g provides us a new way of reorganizing the data.

Shrink command: This command is only applicable for tables which are tablespace with auto segment space management.
Before using this command, you should have row movement enabled.

SQL> alter table test enable row movement;
Table altered.

There are 2 ways of using this command.

1. Break in two parts: In first part rearrange rows and in second part reset the HWM.

Part 1: Rearrange (All DML's can happen during this time)

SQL> alter table test shrink space compact;
Table altered.

Part 2: Reset HWM (No DML can happen. but this is fairly quick, infact goes unnoticed.)
SQL> alter table sa shrink space;
Table altered.

2. Do it in one go:

SQL> alter table sa shrink space; (Both rearrange and restting HWM happens in one statement)
Table altered.

Few advantages over the conventional methods

1. Unlike "alter table move ..", indexes are not in UNUSABLE state. After shrink command, indexes are updated also.
2. Its an online operation, So you dont need downtime to do this reorg.
3. It doesnot require any extra space for the process to complete.

Conclusion
Its a new 10g feature to shrink (reorg) the tables (almost) online which can be used with automatic segment space management.