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Thursday, November 1, 2007

Materialized view for Data Warehouse

Dear friends,

Did you hear about Materialized view?
Ans: Yes, No or May be.

Now I will explain you in details

What is materialized view?

1. Materialized view is normal database object like “table,index”
2. It is basically use for Data warehouse or Replication purpose.
3. Snapshot is synonym for materialized view.
4. A materialized view can be stored in the same database as its base tables or in a different database
5. A materialized view provides access to table data by storing the results of a query in a separate schema object. Unlike an ordinary view, which does not take up any storage space or contain any data, a materialized view contains the rows resulting from a query against one or more base tables or views.
6. A materialized view improve response time through query rewrite or reduce execution time.

Note: Query rewrite is a mechanism where Oracle or applications from the end user or database transparently improve query response time, by automatically rewriting the SQL query to use the materialized view instead of accessing the original tables. Query rewrites are particularly useful in a data warehouse environment.

What is syntax for materialized view?

Create materialized view “materialized_view_name”
Build [immediate / deferred]
Refresh [fash [on commit / demand]]
Enable query rewrite
As
Sql statement
/

Now Details explanation about syntax
1. Create materialized view “name’
I hope it is understood because self explanatory

2. Build [immediate / deferred]
1. Build immediate -- > materialized view populate with data on creation time from tables.
2. Build deferred --- > materialized view not populate with data on creation time, we have to populate later.

Eg:
With build immediate



create materialized view mymview1
build immediate
as
select ename,sum(sal) from emp group by ename;

SQL> select count(*) from emp;

COUNT(*)
----------
14

Note: materialized view populate with data on creation time.

With build deferred
create materialized view mymview2
build deferred
as
select ename,sum(sal) from emp group by ename;

SQL> select count(*) from mymview2;

COUNT(*)
----------
0

Note: materialized view not populate with data on creation time, we have to populate manually through DBMS_MVIEW.RERESH package.


3. Refresh [fast [on commit / demand]]
Refresh is method for populate materialized view with data whenever made changes in their base tables.

There are five refresh method for materialized view
1. refresh fast (materialized view log needed)
2. refresh on commit
3. refresh on demand

What is materialized view log?
A materialized view log is a schema object that records changes to a master table's data so that a materialized view defined on the master table can be refreshed incrementally.
Each materialized view log is associated with a single master table. The materialized view log resides in the same database and schema as its master table.

Eg:
With refresh on commit


SQL> select count(*) from emp;
COUNT(*)
----------
14
SQL> create materialized view MYVIEW1
2 build immediate
3 refresh on commit
4 enable query rewrite
5 as
6 select ename, sum(sal) from emp group by ename;
Materialized view created.

SQL> select count(*) from myview1;
COUNT(*)
----------
14
SQL> insert into emp (empno,ename,sal) values(1754,'TAJ',2500);
1 row created.
SQL> commit;
Commit complete.
SQL> select count(*) from myview1;
COUNT(*)
----------
15
With refresh on commit
SQL> select count(*) from emp;

COUNT(*)
----------
15

SQL> create materialized view MYVIEW5
2 build immediate
3 refresh on demand
4 enable query rewrite
5 as
6 select ename,sum(sal) from emp group by ename;

Materialized view created.

SQL> select count(*) from myview5;

COUNT(*)
----------
15

SQL> insert into emp (empno,ename,sal)values(1100,'xyz',1000);

1 row created.

SQL> commit;

Commit complete.

SQL> select count(*) from emp;

COUNT(*)
----------
16

SQL> select count(*) from myview5;

COUNT(*)
----------
15

SQL> exec dbms_mview.refresh('MYVIEW5');

PL/SQL procedure successfully completed.

SQL> select count(*) from myview5;

COUNT(*)
----------
16

Whenever any changes made in base tables and perform commit then materialized view refreshed.

4. Enable query rewrite
A materialized view is like a query with a result that is materialized and stored in a table. When a user query is found compatible with the query associated with a materialized view, the user query can be rewritten in terms of the materialized view. This technique improves the execution of the user query, because most of the query result has been precomputed. The query transformer looks for any materialized views that are compatible with the user query and selects one or more materialized views to rewrite the user query. The use of materialized views to rewrite a query is cost-based. That is, the query is not rewritten if the plan generated without the materialized views has a lower cost than the plan generated with the materialized views.
Note: query_rewrite_enabled=true AND query_rewrite_integrity=enforced must be set to use query rewrite feature

5. How Materialized view improve query performance.

Let’s check with demo

1. Create Big Table :)
SQL> create table big
2 as select * from all_objects
3 union all
4 select * from all_objects
5 union all
6 select * from all_objects
7 union all
8 select * from all_objects
9 union all
10 select * from all_objects;

Table created.

SQL> select count(*) from big;

COUNT(*)
----------
188995

2. Now execute below query and check query statistics
SQL> alter system flush shared_pool;

System altered.

SQL> set autotrace traceonly
SQL> set timing on
SQL> select owner,count(*) from big group by owner;

12 rows selected.

Elapsed: 00:00:00.01

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=624 Card=188024 By
tes=3196408)

1 0 SORT (GROUP BY) (Cost=624 Card=188024 Bytes=3196408)
2 1 TABLE ACCESS (FULL) OF 'BIG' (TABLE) (Cost=598 Card=1880
24 Bytes=3196408)





Statistics
----------------------------------------------------------
957 recursive calls
0 db block gets
2844 consistent gets---- > too big :)
0 physical reads
0 redo size
1028 bytes sent via SQL*Net to client
504 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
10 sorts (memory)
0 sorts (disk)
12 rows processed

3. Now Create materialized view and try
SQL> create materialized view MYVIEWBIG
2 build immediate
3 refresh on commit
4 enable query rewrite
5 as
6 select owner,count(*) from big group by owner;

Materialized view created.

SQL> alter system flush shared_pool;

System altered.

SQL> set autotrace traceonly
SQL> set timing on
SQL> select owner,count(*) from big group by owner;

12 rows selected.

Elapsed: 00:00:00.01

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=3 Card=12 Bytes=36
0)

1 0 MAT_VIEW REWRITE ACCESS (FULL) OF 'MYVIEWBIG' (MAT_VIEW RE
WRITE) (Cost=3 Card=12 Bytes=360)





Statistics
----------------------------------------------------------
7356 recursive calls
0 db block gets
1313 consistent gets --- > just less then half :)
0 physical reads
0 redo size
1028 bytes sent via SQL*Net to client
504 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
49 sorts (memory)
0 sorts (disk)
12 rows processed


4. What Privileges Need for materialized view ?

To create a materialized view in your own schema, you must have the CREATE MATERIALIZED VIEW privilege and the SELECT privilege to any tables referenced that are in another schema. To create a materialized view in another schema, you must have the CREATE ANY MATERIALIZED VIEW privilege and the owner of the materialized view needs SELECT privileges to the tables referenced if they are from another schema. Moreover, if you enable query rewrite on a materialized view that references tables outside your schema, you must have the GLOBAL QUERY REWRITE privilege or the QUERY REWRITE object privilege on each table outside your schema.

5. How to check Status of a Materialized View?

SQL> select mview_name,query,rewrite_enabled,refresh_mode,build_mode,staleness,c
ompile_state
2 from user_mviews;

MVIEW_NAME QUERY R REFRES BUILD_MOD STALENESS COMPILE_ST
---------- -------------------- - ------ --------- ---------- ----------

MYBIG select owner, count( Y DEMAND IMMEDIATE NEEDS_COMP NEEDS_COMP
*) from big group by ILE ILE
owner

MYMVIEW2 select ename,sum(sal N DEMAND DEFERRED UNUSABLE NEEDS_COMP
) from emp group by ILE
ename

MYVIEW5 select ename,sum(sal Y DEMAND IMMEDIATE FRESH VALID
) from emp group by
ename

MYMVIEW3 select ename,sum(sal Y COMMIT IMMEDIATE UNUSABLE VALID
) from emp group by
ename

MYVIEW1 select ename, sum(sa Y COMMIT IMMEDIATE FRESH VALID
l) from emp group by

Explanation:
MVIEW_NAME -------- > Materialized View name
QUERY ------------- > Query that defines the materialized view
REFRESH_ENABLED --- > If query_rewrite_enabled=TRUE then value is “Y” otherwise “N”
REFRESH_MODE ------ > DEMAND, COMMIT, NEVER
BUILD_MODE -------- > IMMEDIATE, DEFERRED
STALENESS --------- > Relationship between the contents of the materialized view and the contents of the materialized view's masters.
COMPILE_STATE ----- > Validity of the materialized view with respect to the objects upon which it depends
VALID -> No objects depends has changed since the last validation
NEED_COMPILE -> Some objects depends materialized view has changed an “alter materialized view … compile;” statement is required to validate this materialized view

3 comments:

Anonymous said...

The best explanation of materialized views I have ever seen. Thank you so much!

ptrikha1 said...

Great explanation- I had read about Materialized View- but this simple explanation makes it so lucid and clear. Hats off to you!

ptrikha1 said...

Hats off to you for such a crystal clear and simple explanation!