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Temporary Tablespace Groups

Oracle 10g introduced the concept of temporary tablespace groups.

These allow to group multiple temporary tablespaces into a single group and assign a user this group of tablespaces instead of a single temporary tablespace.

This raises some interesting questions, and for some of these I don't find answers in the official documentation. Some of these questions are:

- Can a single workarea execution allocate space from more than one temporary tablespace, e.g. to support large serial sort operations?

A workarea belongs to a single operation of an execution plan. There are several different types of operations that require a workarea, among them are sorts, hash joins, group bys and sort/merge joins.

This workarea can fit into available PGA memory, but can also spill to disk in case there is insufficient memory available to support the operation.

Furthermore this implies that a execution of a single SQL statement can require multiple workareas, e.g. a quite simple statement might need two workareas for two hash joins and a third one for a subsequent sort order by operation.

Note that there are other types of operations that don't require a workarea, e.g. a nested loop join doesn't require a workarea (and therefore will never acquire temporary space).

Details about workareas can be obtained from various dynamic performance views, e.g. V$SQL_WORKAREA, V$SQL_WORKAREA_ACTIVE and V$SQL_WORKAREA_HISTOGRAM.

- Can multiple workareas of a single session allocate space from different tablespaces?

- According to the documentation different sessions of the same user can use different temporary tablespaces from the group. Is this correct?

- What about parallel execution? The documentation says that the parallel slaves can use different temporary tablespaces. Is this correct?

Here's a simple testcase that generates two small temporary tablespaces and assigns them to the same group. A test user is assigned first a single temporary tablespace and afterwards the group to see if this makes any difference.

It uses a carefully crafted table for which a sort operation doesn't fit into a single small temporary tablespace but is supposed to fit if both temporary tablespaces of the group can be used.

The session is deliberately using a very small sort_area_size in manual workarea policy mode to force the sort operation to spill to disk.

The following are the results from 11.1.0.7 Win32. Similar results can be seen from 10.2.0.4 Win32. The database used a 8KB default block size and a locally managed tablespace with manual segment space management (no ASSM) for the data but that shouldn't matter here much.

SQL>
SQL> drop tablespace temp1_grp including contents and datafiles;

Tablespace dropped.

SQL>
SQL> drop tablespace temp2_grp including contents and datafiles;

Tablespace dropped.

SQL>
SQL> create temporary tablespace temp1_grp
2 tempfile 'C:\ORACLE\ORADATA\ORCL11\TEMP1_GRP.DBF'
3 size 2M reuse uniform size 100K
4 tablespace group temp_grp;

Tablespace created.

SQL>
SQL> create temporary tablespace temp2_grp
2 tempfile 'C:\ORACLE\ORADATA\ORCL11\TEMP2_GRP.DBF'
3 size 2M reuse uniform size 100K
4 tablespace group temp_grp;

Tablespace created.

SQL>
SQL> drop user tempgrp_user cascade;

User dropped.

SQL>
SQL> create user tempgrp_user identified by tempgrp_user;

User created.

SQL>
SQL> grant create session to tempgrp_user;

Grant succeeded.

SQL>
SQL> exec dbms_random.seed(0)

PL/SQL procedure successfully completed.

SQL>
SQL> drop table test_temp_grp purge;

Table dropped.

SQL>
SQL> create table test_temp_grp
2 as
3 select
4 sys.dbms_random.string('U', 140) as object_name
5 from
6 dual
7 connect by
8 rownum <= 10000;

Table created.

SQL>
SQL> exec dbms_stats.gather_table_stats(null, 'TEST_TEMP_GRP')

PL/SQL procedure successfully completed.

SQL>
SQL> select blocks from dba_tables where table_name = 'TEST_TEMP_GRP';

BLOCKS
----------
205

SQL>
SQL> grant select on test_temp_grp to tempgrp_user;

Grant succeeded.

SQL>
SQL> create or replace synonym tempgrp_user.test_temp_grp for test_temp_grp;

Synonym created.

SQL>
SQL> -- This is the behaviour when using traditional tablespaces
SQL> alter user tempgrp_user temporary tablespace temp1_grp;

User altered.

SQL>
SQL> connect tempgrp_user/tempgrp_user
Connect durchgeführt.
SQL>
SQL> alter session set nls_language = 'AMERICAN';

Session altered.

SQL>
SQL> variable r refcursor
SQL>
SQL> variable t refcursor
SQL>
SQL> -- Force sort on disk
SQL> alter session set workarea_size_policy = manual sort_area_size = 65536;

Session altered.

SQL>
SQL> exec open :r for select * from test_temp_grp order by object_name

PL/SQL procedure successfully completed.

SQL>
SQL> -- Sort doesn't fit into available temp space
SQL> declare
2 rec test_temp_grp%rowtype;
3 begin
4 fetch :r into rec;
5 end;
6 /
declare
*
ERROR at line 1:
ORA-01652: unable to extend temp segment by 13 in tablespace TEMP1_GRP
ORA-06512: at line 4

SQL>
SQL> exec open :r for select * from test_temp_grp where rownum <= 5000 order by object_name

PL/SQL procedure successfully completed.

SQL>
SQL> -- This works
SQL> declare
2 rec test_temp_grp%rowtype;
3 begin
4 fetch :r into rec;
5 end;
6 /

PL/SQL procedure successfully completed.

SQL>
SQL> connect cbo_test/cbo_test
Connect durchgeführt.
SQL>
SQL> alter session set nls_language = 'AMERICAN';

Session altered.

SQL>
SQL> -- Switch to tablespace group
SQL> alter user tempgrp_user temporary tablespace temp_grp;

User altered.

SQL>
SQL> connect tempgrp_user/tempgrp_user
Connect durchgeführt.
SQL>
SQL> alter session set nls_language = 'AMERICAN';

Session altered.

SQL>
SQL> -- Force sort on disk
SQL> alter session set workarea_size_policy = manual sort_area_size = 65536;

Session altered.

SQL>
SQL> exec open :r for select * from test_temp_grp order by object_name

PL/SQL procedure successfully completed.

SQL>
SQL> -- Sort still doesn't fit into available temp space (but it should as you'll see later)
SQL> -- so obviously a single workarea can't use space from different tablespaces
SQL> declare
2 rec test_temp_grp%rowtype;
3 begin
4 fetch :r into rec;
5 end;
6 /
declare
*
ERROR at line 1:
ORA-01652: unable to extend temp segment by 13 in tablespace TEMP1_GRP
ORA-06512: at line 4

SQL>
SQL> -- What if we have multiple workareas per session
SQL> exec open :r for select * from test_temp_grp where rownum <= 5000 order by object_name

PL/SQL procedure successfully completed.

SQL>
SQL> -- First one works
SQL> declare
2 rec test_temp_grp%rowtype;
3 begin
4 fetch :r into rec;
5 end;
6 /

PL/SQL procedure successfully completed.

SQL>
SQL> exec open :t for select * from test_temp_grp where rownum <= 5000 order by object_name

PL/SQL procedure successfully completed.

SQL>
SQL> -- Second one fails
SQL> declare
2 rec test_temp_grp%rowtype;
3 begin
4 fetch :t into rec;
5 end;
6 /
declare
*
ERROR at line 1:
ORA-01652: unable to extend temp segment by 13 in tablespace TEMP1_GRP
ORA-06512: at line 4

SQL>
SQL> -- So it looks like a session is limited to a particular tablespace taken from the group
SQL> -- What happens to a second session of the same user
SQL> -- One tablespace is still in use by cursor R
SQL> -- Check V$SORT_USAGE if interested
SQL>
SQL> -- Run the same query in second session
SQL> -- You might need multiple attempts to get this working
SQL> -- Depending on the tablespace taken from the group
SQL> -- and then close second session to free temp space
SQL> -- Press ENTER to continue
SQL>
SQL> pause

SQL>
SQL> -- This works if the second session is assigned a different tablespace taken from the group
SQL> -- So different sessions of the same user will potentially be assigned to different tablespaces
SQL>
SQL> -- What about parallel execution
SQL> exec close :r

PL/SQL procedure successfully completed.

SQL>
SQL> exec open :r for select /*+ parallel(test_temp_grp, 2) */ * from test_temp_grp order by object_name

PL/SQL procedure successfully completed.

SQL>
SQL> -- This works
SQL> -- So the PX sessions can be assigned to different tablespaces taken from the group
SQL> -- Check V$SORT_USAGE if interested
SQL> declare
2 rec test_temp_grp%rowtype;
3 begin
4 fetch :r into rec;
5 end;
6 /

PL/SQL procedure successfully completed.

SQL>
SQL> spool off

So in summary the following can be deduced:

1. A single workarea execution can't use space from more a single tablespace even when the user is assigned a temporary tablespace group. Or in other words: A single workarea execution can only allocate a single temporary segment, and segments in general can't span multiple tablespaces in Oracle, i.e. they must fit into a single tablespace.

2. Even multiple workareas for a single session can't use space from the multiple temporary tablespaces available. So this looks like a session attribute, i.e. assigned to the session, and all workareas of that session will use the single temporary tablespace assigned (taken from the group)

3. Multiple sessions of the same user can be assigned to different temporary tablespaces taken from the group.

4. Parallel execution slave sessions can do the same, they can be assigned to different temporary tablespaces taken from the group.

So in order to leverage the power of temporary tablespace groups you need either multiple sessions running serial SQL or the parallel execution option.

User objects created in the SYS schema and the (cost based) optimizer

The answer - as already disclosed by Nicolas Gasparotto - to the question that I asked here in the "Weekend Quiz" is to run the script as SYS user, and then run the query shown against these objects in the SYS schema (tested against 10g XE, 10.2.0.4 and 11.1.0.7 on Win32).

Note: It's not recommended to create any non-SYS objects in the SYS schema and you should only perform this (if at all) in a test database.

All this came up in this recent OTN forum thread where it became obvious that the issue can only be reproduced if the objects are owned by SYS.

There are two interesting points to derive from this (apart from the obvious that one should not create any user objects in the SYS schema):

1. The optimizer seems to treat objects owned by SYS differently, in particular regarding the transformations applied. Note that the crucial point is not that the query is executed as SYS user, but that the objects are owned by the SYS user. Granting appropriate privileges to a non-SYS user on the objects owned by SYS allows to reproduce the issue even with a non-SYS user.

2. It's something to remind if there is the need to understand a performance issue with a recursive dictionary query performed on SYS-owned objects. Although you obviously can't influence the SQL generated by Oracle itself it might help to understand the issue and take appropriate steps to rectify the issue.

Oh, by the way, have I already mentioned that it's really a bad idea to create user objects in the SYS schema?

Weekend quiz

Quiz questions seem to be in nowadays, so here's one for you regarding the cost based optimizer:

Given this simple script (Run this on any 10.2.x (including XE) or 11.1.0.6/7):

create table test1
as
select * from dba_objects;

create table test2
as
select * from dba_objects;

create index i_test1_1 on test1(object_name);

create index i_test1_2 on test1(object_id);

create index i_test2_1 on test2(object_name);

create index i_test2_2 on test2(object_id);

exec dbms_stats.gather_table_stats(null, 'TEST1', estimate_percent=>null, method_opt=>'for all columns size 1');

exec dbms_stats.gather_table_stats(null, 'TEST2', estimate_percent=>null, method_opt=>'for all columns size 1');

And this simple query:

explain plan for
select
*
from
test1
where
object_name='TEST1'
and object_id in (
select /*+ unnest */
object_id
from
test2
);

with this plan:

Plan 1 (the obvious one):

Plan hash value: 2107173885

------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2 | 212 | 6 (0)| 00:00:01 |
| 1 | NESTED LOOPS SEMI | | 2 | 212 | 6 (0)| 00:00:01 |

Read Consistency, "ORA-01555 snapshot too old" errors and the SCN_ASCENDING hint

Oracle uses for its read consistency model a true multi-versioning approach which allows readers to not block writers and vice-versa, writers to not block readers. Obviously this great feature allowing highly concurrent processing doesn't come for free, since somewhere the information to build multiple versions of the same data needs to be stored.

Oracle uses the so called undo information not only to rollback on-going transactions but also to re-construct old versions of blocks if required. Very simplified when reading data Oracle knows the point in time (which corresponds to an internal counter called SCN, System Change Number) that data needs to be consistent with. In the default READ COMMITTED isolation mode this point in time is defined when a statement starts to execute. You could also say at the moment a statement starts to run its result is pre-ordained. When Oracle processes a block it checks if the block is "old" enough and if it discovers that the block content is too new (has been changed by other sessions but the current access is not supposed to see this updated content according to the point-in-time assigned to the statement execution) it will start to create a copy of the block and use the information available from the corresponding undo segment to re-construct an older version of the block. Note that this process can be iterative: If after re-constructing the older version of the block it's still not sufficiently old more undo information will be used to go further back in time.

Optimizer partition oddities, part 2: List partitioning

Back to part 1

Some time ago on the OTN forum the following table layout was part of a discussion regarding performance issues and it revealed an interesting anomaly regarding list partition pruning:

If you're using list partitioning with partitions that use multiple values that map to a single list partition then the optimizer obviously uses a questionable approach when you're using multiple values on the partition key to prune to a single partition.

Consider the following table layout:

CREATE TABLE XYZ
(
TICKER VARCHAR2(22 BYTE) NOT NULL,
EXCH_CODE VARCHAR2(25 BYTE) NOT NULL,
ID_ISIN VARCHAR2(12 BYTE),
HIGH_52WEEK NUMBER(28,10),
LOW_52WEEK NUMBER(28,10),
PX_OPEN NUMBER(28,10),
PX_HIGH NUMBER(28,10),
BLOOMBERG_FILE_SOURCE VARCHAR2(100 BYTE),
LATEST_VERSION_FLAG CHAR(1 BYTE)
)
PARTITION BY LIST (EXCH_CODE)
(
PARTITION BBO_ASIA VALUES ('SL','IS','SP','JF','JN','PK','KP','VM','JS','IN','TB','KQ','JP','NV','JJ','MK','HK','IJ','JT','TT','PA','CS','JX',
'IB','AU','FS','VN','NZ','KS','PM','CH','BD','JQ','VH','CG','JO')
,
PARTITION BBO_NAMR VALUES ('UO','US','UN','PQ','TR','UD','UP','TX','UL','UB','UU','UX','UT','TN','UQ','UR','UW','UV','TA','CT','CV','UC','CJ','UA','UM','CN','UF','CF')
,
PARTITION BBO_LAMR VALUES ('AR','BS','AC','CR','EG','EK','VB','BN','EQ','PE','AF','CX','KY','CC','MM','BM','TP','BV','BH','UY','BZ','ED','VC','VS','BO','CI','CB','PP','BA','JA','CE')
,
PARTITION BBO_EURO VALUES (DEFAULT)
);

I'm now going to populate that table using this sample data:

OOW'08 Oracle 11g New Features for DBAs

It was my second session at Open World this year. It was full with 332 attendees with a whopping 277 attendees on wait list! the room capacity was 397. Of course, the room did have some fragmentation and not everyone could make it.

Here is the abstract:

There is a world outside the glittering marketing glitz surrounding Oracle 11g. In this session, a DBA and author of the popular 11g New Features series on OTN covers features that stand out in the real world and make your job easier, your actions more efficient and resilient, and so on. Learn the new features with working examples: how to use Database Replay and SQL Performance Analyzer to accurately predict the effect of changes and Recovery Manager (RMAN) Data Recovery Advisor to catch errors and corruption so new stats won't cause issues.

Thank you very much for those who decided to attend. I hope you found it useful. Here is the presentation. You can download it from the Open World site too. Please note, the companion site to see al working examples and a more detailed coverage is still my Oracle 11g New Features Series on Oracle Technology Network.