Materilized Views

Dear All,
Can you tell me exactly what is the difference between Materilized View and snap shots.
when to opt for materilized views.
Regards
VNK

Hello VNK,
despite the changes in handling and maintainability of materialized views over the past db versions,
the main difference lies in a feature called 'query rewriting'.
When adding a materialized view to a schema, the cost based optimizer is able to decide
whether to select from the materialized view or the underlying tables without a change in the application code.
MV's are generally used in DSS / Data Warehousing environments rather than in OLTP applications.
Bernd

Similar Messages

Delete vs truncate on materilized view

is it possible to truncate materilized view? if yes, is it faster than using delete from materialized view?
thanks.

user8837158 wrote:
is it possible to truncate materilized view? YES, for example;
dbms_mview.refresh( 'MY_MVIW_NAME' , 'C', atomic_refresh => FALSE ); => This does a TRUNCATE
if yes, is it faster than using delete from materialized view? Yes it is faster ; TRUNCATE is a DDL, no need as much undo as a DELETE, where a DELETE is a DML
DELETE operation will cause all DELETE triggers on the table to fire. Where with TRUNCATE no triggers will be fired.
thanks.

Regarding Materilized view

What is materilzed view and how it differ it from view why we use and in which cases we have to use the materilized Can anybody help me ?

user13632611 wrote:
What is materilzed view and how it differ it from view why we use and in which cases we have to use the materilized Can anybody help me ?
Overview of Materialized Views.
SY.

Help in materilized view???

Hi All:
Can anybody tell why we create materlized view? Whats the difference in syntax and logic in creating simple view and materlized view. I have made many simple view. But this first time I am making materlized vuew. I only know that it decreases the execution time. Please somebody help me in this . Thanks
Ali

Materialized view really gathers data from tables, while simple view is just remembered select stamement. MV's takes space on disk - views aren't. You can use materialized view in distributed databases or for often used joins or goup by selects. Materialized view fastens your queries because when you make select from MV which is for example join of two tables there is no really joining but just reading data from MV. You shold read also sometihng about refreshing MV.

Error creating materilalized view with union all

Could please anybody advice? What I do wrong?
create table test
i number not null,
constraint pk_test primary key( i ));
create materialized view log on test with rowid;
create materialized view mvtest refresh fast on demand as
SELECT i, 1 umarker FROM test where i < 10000
UNION ALL SELECT i, 2 umarker FROM test where i > 10000;
ORA-12052: cannot fast refresh materialized view MVTEST
It looks like I accomplished all fast refresh requirements, didn't I?
Thanks a lot.
Alexander.

SQL> create table test
2 (i number not null,
3 constraint pk_test primary key( i ));
Table created.
SQL> create materialized view log on test with rowid;
Materialized view log created.
SQL> create materialized view mvtest refresh fast on demand as
2 SELECT t.i, 1 umarker, t.rowid row_id FROM test t where t.i <= 10000
3 UNION ALL
4 SELECT t.i, 2 umarker, t.rowid row_id FROM test t where t.i > 10000;
Materialized view created.(This is on 11.2)

Is materilized view good in this case?

i have two database across different network. I need to replicate couple of tables from main machine to another. This "refresh" has to be every 5 seconds or so.. Do you think it's good idea to have materized view this 5 seconds refresh or any other better option. Tables are not that big .. only 50 to 60 thousand rows in each with 50 columns ...
thanks in advance

This "refresh" has to be every 5 seconds or so.I don't know if a MV is well for a refresh too often like this. Take care to the time of refresh which should less than 5 second, if not, the refresh will occur permanently.
Are you sure that you want 5 second refresh ?
Why not, in that case, query directly sources tables through the dblink instead of MV ? How often should queried your MVs ?
I would think a little more, before apply MV in your case.
Nicolas.

How to drop Tablespace containg materilized views

Hi friends,
I am trying to drop a tablespace but gives an error
SQL> drop tablespace tabs_name including contents and datafiles cascade constraints;
drop tablespace tabs_name including contents and datafiles cascade constraints
ERROR at line 1:
ORA-23515: materialized views and/or their indices exist in the tablespace
Please tell me the solution
Regards,
Neeraj Goel

ORA-23515: materialized views and/or their indices exist in the tablespace
Cause: An attempt was made to drop a tablespace which contains materialized views and/or their indices.
Action: Drop the materialized views in this tablespace. Also, find indices belonging to materialized views in this tablespace and drop then. Then try dropping the tablespace.
Examples
Dropping a Materialized View: Examples The following statement drops the materialized view emp_data in the sample schema hr:
DROP MATERIALIZED VIEW emp_data;
The following statement drops the sales_by_month_by_state materialized view and the underlying table of the materialized view, unless the underlying table was registered in the CREATE MATERIALIZED VIEW statement with the ON PREBUILT TABLE clause:
DROP MATERIALIZED VIEW sales_by_month_by_state;
http://download-uk.oracle.com/docs/cd/B19306_01/server.102/b14200/statements_8019.htm

ER Database diagrams and materilized views

I would like to be able to diagram materialized views on a database diagram.
Other tools can do this either directly or by exposing the underlying physical table

Hi Pete,
This functionality is not currently available, it is currently be looked at scheduling it for the next major JDeveloper release.
Regards,
Lisa
JDev QA

How to recover definition of materilized view dropped - Recycle Bin

Hi,
Have just dropped a mview by mistake.
Is there any easy way to recover this without going to database backups - e.g recycle bin or anything similar.
Many Thanks

Hi,
Since you did not specify a 4 digit database version, I can only gues that you have a 10g db or up.
With flashback query you might be able to do a :
select * from dba_mviews
as of timestamp sysdate - 1 ;Other wise you can restore the base table from the recyclebin. The VIEW query is lost
Cheers
FJFranken

Refresh time for Materialized View

Hi,
I have created a materialized view and set it to refresh everyday at 4 am . When I check the status of the job , it shows the refresh timing between 20 mins to 4 hours . That means , some times it takes 20 mins to refresh , some times 40 , some time 2 hours and some times up to 4 hours. Any reason for the wide range ? Here is the query for the view.
CREATE MATERIALIZED VIEW BDS_COST_CATEGORY_MV
ORGANIZATION HEAP PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255 NOCOMPRESS LOGGING
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "USERS"
BUILD IMMEDIATE
USING INDEX
REFRESH COMPLETE ON DEMAND START WITH sysdate+0 NEXT ROUND(SYSDATE + 1) + 4/24
USING DEFAULT LOCAL ROLLBACK SEGMENT
DISABLE QUERY REWRITE
AS    SELECT AC.ACCOUNT_MANAGER, AC.ACCOUNT_NUMBER,
                AC.ACCOUNT_NAME, DIV.DIVISION_NUMBER, ITEM.CODE_ID,
                ITEM.CODE_NAME COST_CATEGORY_ITEM,
                TO_NUMBER(TO_CHAR(AP.FIRST_DAY,'YYYY')) PERIOD_YEAR,
                AP.FIRST_DAY PERIOD,
                SUM(NVL(AD.CURRENT_EXPENSE,0)) EXPENSE
        FROM ACCOUNTS AC, PERSONNEL AC_MGR, DIVISIONS DIV, ASR_HEADERS AH , ASR_DETAILS AD, CODES ITEM, CODES COSTCAT, ALL_PERIODS AP,
                CODES COST_ELEMENT, ACCOUNTING_BASE_GROUP ABG
        WHERE   AC.ACCOUNT_MANAGER = AC_MGR.PERSONNEL_ID
        AND     AC_MGR.DIVISION_ID = DIV.DIVISION_ID (+)
        AND     AC.ACCOUNT_ID = AH.ACCOUNT_ID
        AND     AH.ASR_HEADER_ID = AD.ASR_HEADER_ID
        AND     AH.PERIOD_ASR = AP.FISCAL_SYSTEM_PERIOD_CAL_YR_EQ||', '|| AP.FISCAL_YEAR
        AND     ABG.COST_ELEMENT_CAT_ITEM_ID = ITEM.CODE_ID
        AND     COST_ELEMENT.CODE_ID = ABG.COST_ELEMENT_ID
        AND     AD.OBJECT_CODE = COST_ELEMENT.CODE_VALUE
        AND     ITEM.CODE_PARENT_ID = COSTCAT.CODE_ID
        AND     ( COSTCAT.CODE_NAME = 'BDS' OR ( COSTCAT.CODE_NAME = 'Base' AND ITEM.CODE_NAME = 'MTDC'))
        GROUP BY AC.ACCOUNT_MANAGER,AC.ACCOUNT_NUMBER, AC.ACCOUNT_NAME,
                 DIV.DIVISION_NUMBER, ITEM.CODE_ID, ITEM.CODE_NAME,
                 TO_NUMBER(TO_CHAR(AP.FIRST_DAY,'YYYY')), AP.FIRST_DAY

This appears to be a duplicate of Refreshing Materilized view. I replied on the other thread.
Justin

How to create an incremental key in a Matrialized View created from Union?

I am trying to create some sort of incremental key in a materialized view that is formed from several Select and Union statements.
Why? My materialized view has a composite inteligent key of 5 columns. So, when I want to find a particular records, I don't want to have to do a select statement with some many WHERE and AND clauses.
The ROWNUM I tried didn't work (as I thought) because my Materialized view is made out of several union and each select statement of the UNION generates a new set of incremental numbers.
I also tried creating a SEQUENCE (I understand the sequence number will keep growing each time I do a refresh ) but it doesn't look like I can create a SEQUENCE on Matrialized View.
Anyone has any suggestion as how do I create some kind of incremental number in a materilized view created from an Union.
Thank you.
H

I'm assuming that this is a continusation of your earlier message
Re: Can I create Sequence on Materialized Views?
It doesn't seem to make sense, as others mentioned in that thread, to try to add this sort of key to a materialized view. Since the key would change every time you refreshed the materialized view, I'm having difficulty understanding how this will simplify your life. Your code would still have to refer to all 5 columns of the composite key since that would be the only constant.
Justin

Why log or not a materializad view ?

Hi,
I know we can log a materialized view but not found a reason to do it. I'm not talking about materilized view logs used to refresh a mv but to log a mv.
If I work with two databases (a - master and b - mv site) and do a lot of changes to a table on database a replicated on database b. Changes are logged on both sides and to my opinion wasting space on database b. Case 2, if do a complete refresh the deleted and inserted data will be logged using sometimes a lot of disk space.
Why my question, I'm used to turn off logs on materilized views but now I'm working with some databases as and want to propose not to log mv because it's usual to have disk space problems.
What do you think about ?

What, exactly, do you mean by "log a materialized view" if not the presence of materialized view logs? Are you talking about
CREATE MATERIALIZED VIEW
LOGGING | NOLOGGINGIf so, that merely controls whether direct-path inserts to the materialized view or the initial materialized view creation generates redo. If you are doing incremental refreshes, NOLOGGING is irrelevant because the refresh mechanism doesn't use direct-path operations. If there are any indexes on the materialized view, NOLOGGING is less relevant because index operations are always logged, so you lose most of the performance benefits.
If you are doing complete refreshes of materialized views and those materialized views are not in refresh groups, Oracle will do direct-path loads and NOLOGGING can be quite useful assuming there are no indexes. There are obviously downsides from a recovery standpoint, however, since those are unrecoverable operations. It may also be useful when initially building a large materialized view to use NOLOGGING.
Justin

Materialized view with aggregates doing a fast refresh

why is that i need to have count(*),count(<expressions used>) in my Materialized view Query with Aggregates?
say mat view query is:
select deptno,sum(sal) from emp,dept where emp.deptno,dept.deptno group by dname.cant do a fast refresh.
BUT
select deptno,sum(sal),count(*),count(sal) from emp,dept where emp.deptno,dept.deptno group by dname.Does a fast refresh.Why?
Also its mentioned in manuals that count(*) and count(expr) is needed but it doesnt explain why.
Thanks

Thanks for the correction.I just wanted to simulate the query and it was a typing mistake.sorry for that.
My query working fine with count(). If i understand it correctly it is to determine whether there should be an update or delete to MV in case of say delete on master table.that is, count is decremented on delete and if it becomes 0 then we need to delete that aggregated row from the MV,else it need to be updated even in case of delete.
But this answers why count() is needed for coulmns in group by clause.
Dont really see a need to have count() in case i m updating the measures as materilized view logs should take care of it.

Create Materialized View with Compress clause

Hi,
Oracle 9i R2 has an option to use compress clause while creating table, materialized view or etc. Just wanted some help on tips to be followed during creating a materilized view with compress clause and refresh the same.do i need to follow any special procedures ??? Any help greatly appreciated.
Thanks,
Chak.

I read in the book that while inserting user /+append+/ otherwise table with compress will fail. I am doing materialized view refresh with fast mode and data will be inserted as per logs residing at master site.. while inserting into materialized view, do i have to setup specially since fast refresh is going to insert data into existing materialized view.
Thanks,

Query Rewrite to Utilize Materialized Views on a Remote Database

We have our materilized views stored at a remote database. Is there a way to utilize query rewrite to rewrite the queries issued on the local database such that queries will refer the materialized views at a remote db?
From the documentation, I already know that the base tables for MVs and MVs should eside in the same instance or the query should be submitted on the instance where MVs reside if base tables are residing in a different remote DB to benefit from query rewrite.
My aim is to utilize query rewrite to reduce the workload on a primary database by passing queries that can be rewritten to use MVs on a remote DB.
Thanks for the guidance,

My aim is to utilize query rewrite to reduce the workload on a primary database by passing queries that can be rewritten to use MVs on a remote DB. If you know that MVs are not stale or acceptable of staled data and don't see network bandwidth problems (small set of result), You would try accessing MV tables directly on remote db using dblinks.

Materilized Views

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