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Oracle 10g Segment shrink

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Mandatory

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Init.ora parameter 'Compatible' must be > = 10.0

Shrink operations can be performed only on segments in locally managed

Tablespaces with automatic segment space management (ASSM).

How it works

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1. Enable row movement for the table.

SQL > ALTER TABLE scott.emp ENABLE ROW MOVEMENT

2. Shrink table but don't want to shrink HWM (High Water Mark).

SQL > ALTER TABLE scott.emp SHRINK SPACE COMPACT

3. Shrink table and HWM too.

SQL > ALTER TABLE scott.emp SHRINK SPACE

4. Shrink table and all dependent index too.

SQL > ALTER TABLE scott.emp SHRINK SPACE CASCADE

5. Shrink table under MView.

SQL > ALTER TABLE SHRINK SPACE

6. Shrink Index only.

SQL > ALTER INDEX SHRINK SPACE

Restrictions on the shrink_clause, 10gR1

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1. You cannot specify this clause for a cluster, a clustered table, or any

Object with a LONG column.

2. Segment shrink is not supported for LOB segments even if CASCADE is

Specified.

3. Segment shrink is not supported for tables with function-based indexes.

4. This clause does not shrink mapping tables or overflow segments of

Index-organized tables, even if you specify CASCADE.

5. You cannot shrink a table that is the master table of an ON COMMIT

Materialized view. Rowid materialized views must be rebuilt after the

Shrink operation.

6. Table with a domain index is not supported.

Restrictions on the shrink_clause, 10gR2

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1. You cannot specify this clause for a cluster, a clustered table, or any

Object with a LONG column.

2. Segment shrink is not supported for tables with function-based indexes or

Bitmap join indexes.

3. This clause does not shrink mapping tables of index-organized tables

Even if you specify CASCADE.

4. You cannot specify this clause for a compressed table.

5. You cannot shrink a table that is the master table of an ON COMMIT

Materialized view. Rowid materialized views must be rebuilt after the

Shrink operation.

6. Table with a domain index is not supported.

Restrictions on the shrink_clause, 11gR1

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1. You cannot combine this clause with any other clauses in the same ALTER TABLE

Statement.

2. You cannot specify this clause for a cluster, a clustered table, or any

Object with a LONG column.

3. Segment shrink is not supported for tables with function-based indexes or

Bitmap join indexes.

4. This clause does not shrink mapping tables of index-organized tables, even if

You specify CASCADE.

5. You cannot specify this clause for a compressed table.

6. You cannot shrink a table that is the master table of an ON COMMIT

Materialized view. Rowid materialized views must be rebuilt after the shrink

Operation.

Query/DML Concurrency

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The online phase of segment shrink is done with DML-compatible locks. Hence DMLs

Can coexist during this phase. During the space-release/HWM adjustment phase

Incompatible locks will be acquired on the table, hence, DMLs will block on

Shrink.

There are no user visible errors that shrink will cause on DMLs.

Queries cache the segment HWM. Oracle guarantees that the HWM always moves forward

Hence CR (consistent read) is not required on segment header and extent map blocks. The only operations

That cause the segment HWM to move backward are drop and truncate.

We allow queries to coexist with drop/truncate DDLs since queries do not acquire locks.

If after the drop/truncate, the space gets reused in some other segment, then the

Queries get "8103-object does not exist" external error message.

During segment shrink, the segment thanks to ™s incarnation number is changed in the bitmap

Blocks and segment header when the segment HWM is adjusted. Subsequent data block

Changes happen at this newer incarnation number.

Queries that span this phase can die with an external error "10632-invalid rowid"

If

1) They read the updated bitmap blocks (that have new inc#). Note that this failure

Happens if the space is not reused

2) The space got reused for some other object or the same object.

Limitations on Online Segment Shrink, 10gR2:

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Within an ASSM tablespace, all segment types are eligible for online segment shrink except these:

-IOT mapping tables

-Tables with rowid based materialized views

-Tables with function-based indexes

Dependency Maintenance and Restrictions on Shrink

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The only dependency that will be taken care of during segment shrink is the index.

The indexes will not be in an unusable state after shrink.

The compaction phase of segment shrink will be done as insert/delete pairs. The

DML triggers will not be fired during data movement phase. Since the data does

Not change, it is not required to fire the triggers.

ROWID based triggers should be disabled before issuing a shrink since it will not

Fire during shrink.

Segment shrink cannot be done on objects with on-commit materialized views.

Materialized views that are based on primary key need not be refreshed or rebuilt

After shrink.

However, it is the DBA thanks to ™s responsibility to refresh/rebuild the materialized views

That are rowid based.

Availability

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Segment shrink is done online, thereby it increases the availability of the object.

While conventional DML operations can coexist with segment shrink, parallel DMLs

Cannot.

During segment shrink, data will be moved as part of the compaction phase. During

Compaction locks will be held on individual rows and/or blocks containing the data.

This will cause the concurrent DMLs like updates and deletes to serialize on the

Locks. The compaction will be done in units of smaller transactions, so the availability

Of the object will not be impacted significantly.

However during certain phases of segment shrink (when the HWM is adjusted), the segment

Will have to be locked in exclusive mode.

This phase is for a very short duration and should impact the availability of the

Object less significantly.

Security

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The privileges required to execute segment shrink on an object will be the same

As that for ALTER object.

Detail Example

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SQL > set serveroutput on

SQL > declare

2 v_unformatted_blocks number

3 v_unformatted_bytes number

4 v_fs1_blocks number

5 v_fs1_bytes number

6 v_fs2_blocks number

7 v_fs2_bytes number

8 v_fs3_blocks number

9 v_fs3_bytes number

10 v_fs4_blocks number

11 v_fs4_bytes number

12 v_full_blocks number

13 v_full_bytes number

14 begin

15 dbms_space.space_usage ('SYSTEM',' time shrimp, 'TABLE', v_unformatted_blocks

16 v_unformatted_bytes, v_fs1_blocks, v_fs1_bytes, v_fs2_blocks, v_fs2_bytes

17 v_fs3_blocks, v_fs3_bytes, v_fs4_blocks, v_fs4_bytes, v_full_blocks, v_full_bytes)

18 dbms_output.put_line ('Unformatted Blocks =' | | v_unformatted_blocks)

19 dbms_output.put_line ('FS1 Blocks =' | | v_fs1_blocks)

20 dbms_output.put_line ('FS2 Blocks =' | | v_fs2_blocks)

21 dbms_output.put_line ('FS3 Blocks =' | | v_fs3_blocks)

22 dbms_output.put_line ('FS4 Blocks =' | | v_fs4_blocks)

23 dbms_output.put_line ('Full Blocks =' | | v_full_blocks)

24 end

25 /

Unformatted Blocks = 0

FS1 Blocks = 0

FS2 Blocks = 1

FS3 Blocks = 1

FS4 Blocks = 3

Full Blocks = 0

PL/SQL procedure successfully completed.

SQL > alter table t_shrink shrink space compact

Alter table t_shrink shrink space compact

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ERROR at line 1:

ORA-10636: ROW MOVEMENT is not enabled

SQL > ALTER TABLE t_shrink ENABLE ROW MOVEMENT

Table altered.

SQL > ALTER TABLE t_shrink SHRINK SPACE COMPACT

Table altered.

SQL > set serveroutput on

SQL > declare

2 v_unformatted_blocks number

3 v_unformatted_bytes number

4 v_fs1_blocks number

5 v_fs1_bytes number

6 v_fs2_blocks number

7 v_fs2_bytes number

8 v_fs3_blocks number

9 v_fs3_bytes number

10 v_fs4_blocks number

11 v_fs4_bytes number

12 v_full_blocks number

13 v_full_bytes number

14 begin

15 dbms_space.space_usage ('SYSTEM',' time shrimp, 'TABLE', v_unformatted_blocks

16 v_unformatted_bytes, v_fs1_blocks, v_fs1_bytes, v_fs2_blocks, v_fs2_bytes

17 v_fs3_blocks, v_fs3_bytes, v_fs4_blocks, v_fs4_bytes, v_full_blocks, v_full_bytes)

18 dbms_output.put_line ('Unformatted Blocks =' | | v_unformatted_blocks)

19 dbms_output.put_line ('FS1 Blocks =' | | v_fs1_blocks)

20 dbms_output.put_line ('FS2 Blocks =' | | v_fs2_blocks)

21 dbms_output.put_line ('FS3 Blocks =' | | v_fs3_blocks)

22 dbms_output.put_line ('FS4 Blocks =' | | v_fs4_blocks)

23 dbms_output.put_line ('Full Blocks =' | | v_full_blocks)

24 end

25 /

Unformatted Blocks = 0

FS1 Blocks = 0

FS2 Blocks = 0

FS3 Blocks = 0

FS4 Blocks = 2

Full Blocks = 1

PL/SQL procedure successfully completed.

SQL > ALTER TABLE t_shrink SHRINK SPACE

Table altered.

SQL > declare

2 v_unformatted_blocks number

3 v_unformatted_bytes number

4 v_fs1_blocks number

5 v_fs1_bytes number

6 v_fs2_blocks number

7 v_fs2_bytes number

8 v_fs3_blocks number

9 v_fs3_bytes number

10 v_fs4_blocks number

11 v_fs4_bytes number

12 v_full_blocks number

13 v_full_bytes number

14 begin

15 dbms_space.space_usage ('SYSTEM',' time shrimp, 'TABLE', v_unformatted_blocks

16 v_unformatted_bytes, v_fs1_blocks, v_fs1_bytes, v_fs2_blocks, v_fs2_bytes

17 v_fs3_blocks, v_fs3_bytes, v_fs4_blocks, v_fs4_bytes, v_full_blocks, v_full_bytes)

18 dbms_output.put_line ('Unformatted Blocks =' | | v_unformatted_blocks)

19 dbms_output.put_line ('FS1 Blocks =' | | v_fs1_blocks)

20 dbms_output.put_line ('FS2 Blocks =' | | v_fs2_blocks)

21 dbms_output.put_line ('FS3 Blocks =' | | v_fs3_blocks)

22 dbms_output.put_line ('FS4 Blocks =' | | v_fs4_blocks)

23 dbms_output.put_line ('Full Blocks =' | | v_full_blocks)

24 end

25 /

Unformatted Blocks = 0

FS1 Blocks = 0

FS2 Blocks = 0

FS3 Blocks = 0

FS4 Blocks = 1

Full Blocks = 1

PL/SQL procedure successfully completed.

SQL >

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To monitor the progress of a long running SHRINK operation you may execute the pl/sql block in this note. You should see a change in the number of blocks.

For specific details on the output of the pl/sql blocks in this Note please reference the following documentation:

For 9i:

Http://download.oracle.com/docs/cd/B10501_01/appdev.920/a96612/d_space2.htm#1002701

For 10gR2:

Http://download.oracle.com/docs/cd/B19306_01/appdev.102/b14258/d_space.htm#sthref6026

For 11gR1:

Http://download.oracle.com/docs/cd/B28359_01/appdev.111/b28419/d_space.htm#sthref7301

NOTE:

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Shrinking space of a large segment can take a lot of time, e.g. Tens of hours, and can generate lots of redo.

Therefore, it is not advised to interrupt a shrink statement to prevent a possibly massive rollback.

Best approach would be:

1. Initially, only use:

SQL > alter table shrink space compact

This also takes long but should have little impact because of only short dml locks

2. Only when convenient, issue

SQL > alter table shrink space

Before running in production, it is advised to test for possible impact in your test environment.

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