Is the parallel degree silently ignored? - oracle

Question

My query is the following :
ALTER TABLE EMPLOYEE MODIFY LOB(DOCUMENT) (SHRINK SPACE);
The parallel degree for the table is 1 :
SELECT DEGREE FROM USER_TABLES WHERE TABLE_NAME = 'EMPLOYEE';
Result : 1
Table EMPLOYEE is not partitioned.
If I launch the same query with a parallel degree, the system does not complain, but is it silently ignored ?
ALTER TABLE EMPLOYEE MODIFY LOB(DOCUMENT) (SHRINK SPACE) PARALLEL 8;
Any chance that the query will be faster ?

Answer 1

DDL operations will run in parallel when:
The degree of the table is not 1
OR
The session parallel ddl has been enabled by alter session enable parallel ddl
Anyway, you should always run the alter session enable parallel ddl before running any DDL operation that can run in parallel. Although the documentation did not say that shrink can run in parallel, the syntax is allowed, so I guess you can test whether it runs faster or not.
The parallel DDL statements for nonpartitioned tables and indexes are:
CREATE INDEX
CREATE TABLE AS SELECT
ALTER INDEX REBUILD
The parallel DDL statements for partitioned tables and indexes are:
CREATE INDEX
CREATE TABLE AS SELECT
ALTER TABLE {MOVE|SPLIT|COALESCE} PARTITION
ALTER INDEX {REBUILD|SPLIT} PARTITION
Example
SQL> create table t ( c1 clob ) ;
Table created.
SQL> alter table t MODIFY LOB(c1) (SHRINK SPACE) PARALLEL 8 ;
Table altered.
Instead of SHRINK you can always move the segment lob, which I can assure 100% will run in parallel and faster. The problem is that if you have indexes, there will become invalid.
UPDATE
To move the lob segment, you must do the following
Moving LOB:
SQL> spoolmovelob.sql
SET HEADING OFF
SET pagesize 200
SET linesize 200
select 'ALTER TABLE <owner>.'||TABLE_NAME||' MOVE LOB('||COLUMN_NAME||') STORE AS (TABLESPACE <Tablespace_name>) parallel 5 nologging;' from dba_lobs where TABLESPACE_NAME='<Tablespace_name>';
Note: The above query will include all the LOB,LOBSEGMENT,LOBINDEXES
Moving Table:
SQL> spool /home/oracle/moveTables.sql
SET HEADING OFF
SET PAGESIZE 200
SET LINESIZE 200
select ' ALTER TABLE <owner>.'||TABLE_NAME||' MOVE TABLESPACE <Tablespace_name>) parallel 5 nologging;' from dba_tables where owner='<owner name>';
Moving Indexes:
SQL> spool /home/oracle/moveIndex.sql
SET HEADING OFF
SET long 9999
SET linesize 200
select 'alter index <owner>.'||index_name||' from dba_indexes 'rebuild tablespace <Tablespace_name>) online parallel X nologging;' where owner='<owner>.';
Remember to replace X with the specific degree.

Answer 2

Did you enable parallel ddl?
alter session force parallel ddl parallel 8
Show us your session parameters:
select *
from v$ses_optimizer_env e
where e.sid=userenv('sid')
and (
name like '%parallel%'
or name like '%cpu%'
or name like '%optim%'
)
order by name

Answer 3

Accroding to the Oracle documentation
the ALTER TABLE MODIFY operation cannot be run in parallel.

Answer 4

If I launch the same query with a parallel degree, the system does not complain, but is it silently ignored ?
Depending on how your database in general and your specific seession are configured, it may be ignored.
Any chance that the query will be faster ?
This will very much depend on your specific dataset, your available system resources, and how you set up your parallelism. Pay attention to the parallel_degree_policy setting; it controls default behavior.
See the Oracle whitepaper, "Parallel Execution with Oracle Database" for a more complete understanding. In particular, read the section on "Controlling Parallel Execution" beginning on page 21.