Materialized V

8/3/2019 Materialized V

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Willie Albino May 15, 2003

Materialized Views


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Materialized Views Agenda

What is a Materialized View? Advantages and Disadvantages

How Materialized Views Work Parameter Settings, Privileges, Query Rewrite

Creating Materialized Views Syntax, Refresh Modes/Options, Build Methods

Examples Dimensions

What are they?

Examples


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What is a Materialized View?

A database object that stores the results of a query

Marries the query rewrite features found in Oracle

Discoverer with the data refresh capabilities of snapshots

Features/Capabilities

Can be partitioned and indexed

Can be queried directly Can have DML applied against it

Several refresh options are available

Best in read-intensive environments


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Advantages and Disadvantages

Advantages

Useful for summarizing, pre-computing, replicating and

distributing data

Faster access for expensive and complex joins

Transparent to end-users

MVs can be added/dropped without invalidating coded SQL

Disadvantages

Performance costs of maintaining the views

Storage costs of maintaining the views


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Database Parameter Settings

init.ora parameter COMPATIBLE=8.1.0 (or above)

System or session settings query_rewrite_enabled={true|false}

query_rewrite_integrity={enforced|trusted|stale_tolerated}

Can be set for a session using

alter session set query_rewrite_enabled=true; alter session set query_rewrite_integrity=enforced;

Privileges which must be granted to users directly

QUERY_REWRITE - for MV using objects in own schema

GLOBAL_QUERY_REWRITE - for objects in other schemas


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Query Rewrite Details

query_rewrite_integrity Settings: enforced rewrites based on Oracle enforced

constraints Primary key, foreign keys

trusted rewrites based on Oracle enforced constraintsand known, but not enforced, data relationships

Primary key, foreign keys

Data dictionary information Dimensions

stale_tolerated queries rewritten even if Oracleknows the mvs data is out-of-sync with the detail data

Data dictionary information


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Query Rewrite Details (contd)

Query Rewrite Methods Full Exact Text Match

Friendlier/more flexible version of text matching Partial Text Match

Compares text starting at FROM clause

SELECT clause must be satisfied for rewrite to occur

Data Sufficiency

All required data must be present in the MV or retrievablethrough a join-back operation

Join Compatibility

All joined columns are present in the MV



Query Rewrite Details (contd)

Grouping Compatibility

Allows for matches in groupings at higher levels than

those defined MV query

Required if both query and MV contain a GROUP BYclause

Aggregate Compatibility

Allows for interesting rewrites of aggregations If SUM(x) and COUNT(x) are in MV, the MV may be

used if the query specifies AVG(x)



Syntax For Materialized Views

CREATE MATERIALIZED VIEW

TABLESPACE {}

REFRESH

[ENABLE|DISABLE] QUERY REWRITE

AS

SELECT ;

The determines when MV is built

BUILD IMMEDIATE: view is built at creation time

BUILD DEFFERED: view is built at a later time

ON PREBUILT TABLE: use an existing table as view source

Must set QUERY_REWRITE_INTEGRITY to TRUSTED



Refresh Options

COMPLETE totally refreshes the view

Can be done at any time; can be time consuming

FAST incrementally applies data changes A materialized view log is required on each detail table

Data changes are recorded in MV logs or direct loader logs

Many other requirements must be met for fast refreshes

FORCEdoes a FAST refresh in favor of a COMPLETE The default refresh option

Materialized View Refresh Options



Materialized View Refresh Modes

Refresh Modes

ON COMMIT refreshes occur whenever a commit isperformed on one of the views underlying detail table(s)

Available only with single table aggregate or join based views

Keeps view data transactionally accurate

Need to check alert log for view creation errors

ON DEMAND refreshes are initiated manually using one ofthe procedures in the DBMS_MVIEW package

Can be used with all types of materialized views Manual Refresh Procedures

DBMS_MVIEW.REFRESH(, )

DBMS_MVIEW.REFRESH_ALL_MVIEWS()

START WITH [NEXT] - refreshes start at a specifieddate/time and continue at regular intervals



Materialized View Example

CREATE MATERIALIZED VIEW items_summary_mv

ON PREBUILT TABLE

REFRESH FORCE AS

SELECT a.PRD_ID, a.SITE_ID, a.TYPE_CODE, a.CATEG_ID,

sum(a.GMS) GMS,

sum(a.NET_REV) NET_REV,

sum(a.BOLD_FEE) BOLD_FEE,

sum(a.BIN_PRICE) BIN_PRICE,

sum(a.GLRY_FEE) GLRY_FEE,

sum(a.QTY_SOLD) QTY_SOLD,

count(a.ITEM_ID) UNITS

FROM items a

GROUP BY a.PRD_ID, a.SITE_ID, a.TYPE_CODE, a.CATEG_ID;

ANALYZE TABLE item_summary_mv COMPUTE STATISTICS;



Materialized View Example (contd)

-- Query to test impact of materialized view

select categ_id, site_id,

sum(net_rev),

sum(bold_fee),

count(item_id)

from items

where prd_id in ('2000M05','2000M06','2001M07','2001M08')

and site_id in (0,1)and categ_id in (2,4,6,8,1,22)

group by categ_id, site_id

save mv_example.sql




SQL> ALTER SESSION SET QUERY_REWRITE_INTEGRITY=TRUSTED;

SQL> ALTER SESSION SET QUERY_REWRITE_ENABLED=FALSE;

SQL> @mv_example.sql

CATEG_ID SITE_ID SUM(NET_REV) SUM(BOLD_FEE) COUNT(ITEM_ID)

-------- ------- ------------ ------------- --------------

1 0 -2.35 0 1

22 0 -42120.87 -306 28085

Elapsed: 01:32:17.93

Execution Plan

----------------------------------------------------------

0 SELECT STATEMENT Optimizer=HINT: FIRST_ROWS (Cost=360829 Card=6 Bytes=120)

1 0 SORT (GROUP BY) (Cost=360829 Card=6 Bytes=120)

2 1 PARTITION RANGE (INLIST

3 2 TABLE ACCESS (FULL) OF ITEMS' (Cost=360077

Card=375154 Bytes=7503080)




SQL> ALTER SESSION SET QUERY_REWRITE_ENABLED=TRUE;

SQL> @mv_example.sql

CATEG_ID SITE_ID SUM(NET_REV) SUM(BOLD_FEE) COUNT(ITEM_ID)

-------- ------- ------------ ------------- --------------

1 0 -2.35 0 1

22 0 -42120.87 -306 28085

Elapsed: 00:01:40.47

Execution Plan

----------------------------------------------------------------------------------------------

0 SELECT STATEMENT Optimizer=HINT: FIRST_ROWS (Cost=3749 Card=12 Bytes=276)

1 0 SORT (GROUP BY) (Cost=3749 Card=12 Bytes=276)

2 1 PARTITION RANGE (INLIST)

3 2 TABLE ACCESS (FULL) OF ITEMS_SUMMARY_MV'

(Cost=3723 Card=7331 Bytes=168613)



Example of FAST REFRESH MV

CREATE MATERIALIZED VIEW LOG ON ITEMS

TABLESPACE MV_LOGS STORAGE(INITIAL 10M NEXT 10M) WITH ROWID;

CREATE MATERIALIZED VIEW LOG ON CUSTOMERS

TABLESPACE MV_LOGS STORAGE(INITIAL 1M NEXT 1M) WITH ROWID;

CREATE MATERIALIZED VIEW cust_activity

BUILD IMMEDIATE

REFRESH FAST ON COMMIT

AS

SELECT u.ROWID cust_rowid, l.ROWID item_rowid,

u.cust_id, u.custname, u.email,

l.categ_id, l.site_id, sum(gms), sum(net_rev_fee)

FROM customers u, items l

WHERE u.cust_id = l.seller_id

GROUP BY u.cust_id, u.custname, u.email, l.categ_id, l.site_id;



Getting Information About an MV

Getting information about the key columns of a materialized view:

SELECT POSITION_IN_SELECT POSITION,

CONTAINER_COLUMN COLUMN,DETAILOBJ_OWNER OWNER,

DETAILOBJ_NAME SOURCE,

DETAILOBJ_ALIAS ALIAS,

DETAILOBJ_TYPE TYPE,

DETAILOBJ_COLUMN SRC_COLUMN

FROM USER_MVIEW_KEYS

WHERE MVIEW_NAME=ITEMS_SUMMARY_MV;

POS COLUMN OWNER SOURCE ALIAS TYPE SRC_COLUMN

--- ---------- ----- -------- ----- ------ -----------

1 PRD_ID TAZ ITEMS A TABLE PRD_ID

2 SITE_ID TAZ ITEMS A TABLE SITE_ID

3 TYPE_CODE TAZ ITEMS A TABLE TYPE_CODE

4 CATEG_ID TAZ ITEMS A TABLE CATEG_ID



Getting Information About an MV

Getting information about the aggregate columns of a materializedview:

SELECT POSITION_IN_SELECT POSITION,

CONTAINER_COLUMN COLUMN,

AGG_FUNCTION

FROM USER_MVIEW_AGGREGATES

WHERE MVIEW_NAME=ITEMS_SUMMARY_MV;

POSITION COLUMN AGG_FUNCTION-------- ----------------- ------------

6 GMS SUM

7 NET_REV SUM

: : :

11 QTY_SOLD SUM

12 UNITS COUNT


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Dimensions

A way of describing complex data relationships

Used to perform query rewrites, but not required

Defines hierarchical relationships between pairs of columns

Hierarchies can have multiple levels

Each child in the hierarchy has one and only one parent

Each level key can identify one or more attribute

Child join keys must be NOT NULL

Dimensions should be validated using theDBMS_OLAP.VALIDATE_DIMENSION package

Bad row ROWIDs stored in table: mview$_exceptions


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Syntax For Creating A Dimension

CREATE DIMENSION

LEVEL [ IS

IS ]

HIERARCHY

( CHILD OF

CHILD OF ]

ATTRIBUTE DETERMINES

DETERMINES ,);

To validate a dimension:

exec dbms_olap.validate_dimension(,,FALSE,FALSE);


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Example of Creating A Dimension

CREATE DIMENSION time_dim

LEVEL CAL_DATE IS calendar.CAL_DATE

LEVEL PRD_ID IS calendar.PRD_ID

LEVEL QTR_ID IS calendar.QTR_IDLEVEL YEAR_ID IS calendar.YEAR_ID

LEVEL WEEK_IN_YEAR_ID IS calendar.WEEK_IN_YEAR_ID

HIERARCHY calendar_rollup

(CAL_DATE CHILD OF

PRD_ID CHILD OF

QTR_ID CHILD OF YEAR_ID)

HIERARCHY week_rollup(CAL_DATE CHILD OF

WEEK_IN_YEAR_ID CHILD OF YEAR_ID)

ATTRIBUTE PRD_ID DETERMINES PRD_DESC

ATTRIBUTE QTR_ID DETERMINES QTR_DESC;


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Example of Validating A Dimension

SQL> exec dbms_olap.validate_dimension(time_dim, USER, FALSE, FALSE);

PL/SQL procedure successfully completed.

SQL> select * from mview$_exceptions;

no rows selected.

-- Main cause of errors is a child level having multiple parents

-- If above query returns rows, the bad rows can be found as follows:

select * from calendar

where rowid in

(select bad_rowid from mview$_exceptions);


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Example of Using Dimensions

-- Step 1 of 4

-- Create materialized view (join-aggregate type)

CREATE MATERIALIZED VIEW items_mv

BUILD IMMEDIATE

REFRESH ON DEMAND

ENABLE QUERY REWRITE

AS

SELECT l.slr_id ,

c.cal_date,

sum(l.gms) gms

FROM items l,calendar c

WHERE

l.end_date=c.cal_date

GROUP BY

l.slr_id, c.cal_date;


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Example of Using Dimensions (contd)

-- Step 2 of 4: (not really required, for demonstration only)

-- Execute query based on quarter, not date, without a time dimension

-- Note that the detail tables are accessed

SQL> select c.qtr_id, sum(l.gms) gms

2 from items l, calendar c

3 where l.end_date=c.cal_date

4 group by l.slr_id, c.qtr_id;

Execution Plan

----------------------------------------------------------

SELECT STATEMENT Optimizer=CHOOSE (Cost=16174 Card=36258 Bytes=1160256)SORT (GROUP BY) (Cost=16174 Card=36258 Bytes=1160256)

HASH JOIN (Cost=81 Card=5611339 Bytes=179562848)

TABLE ACCESS (FULL) OF CALENDAR' (Cost=2 Card=8017 Bytes=128272)

TABLE ACCESS (FULL) OF ITEMS' (Cost=76 Card=69993 Bytes=1119888)


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Example of Using Dimensions (contd)

-- Step 3 of 4: Create time dimension (see slide #21 for SQL)

@cr_time_dim.sql

Dimension Created

-- Step 4 of 4: Rerun query based on quarter with time dimension

SQL> select c.qtr_id, sum(l.gms) gms

2 from items l, calendar c

3 where l.end_date=c.cal_date

4 group by l.slr_id, c.qtr_id;

Execution Plan----------------------------------------------------------

SELECT STATEMENT Optimizer=CHOOSE (Cost=3703 Card=878824 Bytes=44820024)

SORT (GROUP BY) (Cost=3703 Card=878824 Bytes=44820024)

HASH JOIN (Cost=31 Card=878824 Bytes=44820024)

VIEW (Cost=25 Card=8017 Bytes=128272)

SORT (UNIQUE) (Cost=25 Card=8017 Bytes=128272)

TABLE ACCESS (FULL) OF CALENDAR (Cost=2 Card=8017 Bytes=128272)

TABLE ACCESS (FULL) OF ITEMS_MV (Cost=3 Card=10962 Bytes=383670)


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Summary

Materialized Views reduce system cpu/io resource requirements by pre-

calculating and storing results of intensive queries allow for the automatic rewriting of intensive queries

are transparent to the application

have storage/maintenance requirements

can understand complex data relationships

can be refreshed on demand or on a schedule

Dimensions allow you to tell Oracle about complex data

relationships which can be used to rewrite queries


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References

Using Oracle9i Materialized Views (Technet Oracle By Example) http://technet.oracle.com/products/oracle9i/htdocs/9iober2/obe9ir2/obe-

dwh/html/mv/mv.htm

Oracle Expert-One-On-One Thomas Kyte

The Secrets of Materialized Views http://www.akadia.com/services/ora_materialized_views.html

OLAP DB-Design with Dimensions http://www.akadia.com/services/ora_olap_dimensions.html

The Secrets of Dimensions http://www.akadia.com/services/ora_dimensions.html
http://technet.oracle.com/products/oracle9i/htdocs/9iober2/obe9ir2/obe-dwh/html/mv/mv.htmhttp://technet.oracle.com/products/oracle9i/htdocs/9iober2/obe9ir2/obe-dwh/html/mv/mv.htmhttp://www.akadia.com/services/ora_materialized_views.htmlhttp://www.akadia.com/services/ora_dimensions.htmlhttp://www.akadia.com/services/ora_dimensions.htmlhttp://www.akadia.com/services/ora_materialized_views.htmlhttp://technet.oracle.com/products/oracle9i/htdocs/9iober2/obe9ir2/obe-dwh/html/mv/mv.htmhttp://technet.oracle.com/products/oracle9i/htdocs/9iober2/obe9ir2/obe-dwh/html/mv/mv.htmhttp://technet.oracle.com/products/oracle9i/htdocs/9iober2/obe9ir2/obe-dwh/html/mv/mv.htm


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Requirements for FAST REFRESH

Requirement Joins Only Joins &Aggregates

Single TableAggregates

Must be based on detail tables only X X X

Must be based on a single table X

Each table can appear only once in the FROM list X X X

Cannot contain nonrepeating expressions (ROWNUM, SYSDATE, etc) X X X

Cannot contain references to RAW or LONG RAW X X X

Cannot contain the GROUP BY clause X

The SELECT list must include the ROWIDs of all the detail tables X

Expressions can be included in the GROUP BY and SELECT clause aslong as they are the same in each

X X

Aggregates are allowed but cannot be nested X X

If SELECT clause contains AVG, it must also contain COUNT X X

If SELECT clause contains SUM, it must also contain COUNT X

If SELECT clause contains VARIANCE, it must also contain COUNTand SUM

X X

If SELECT clause contains STDDEV, it must also contain COUNT andSUM

X

The join predicates of the WHERE clause can included AND but not OR X

The HAVING and CONNECT BY clauses are not allowed X X X


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Rqmts For FAST REFRESH (contd)

Requirement Joins Only Joins &Aggregates

Single TableAggregates

Sub-queries, inline views, or set functions such as UNION are not

allowed

X X X

A WHERE clause is not allowed X

COUNT(*) must be present X

MIN and MAX are not allowed X

Unique constraints must exist on the join columns of the inner table, ifan outer join is used

X

A materialized view log must exist that contains all column referenced inthe materialized view, and it must have been created with the LOGNEW VALUES clause

X

A materialized view log containing ROWID must exist for each detailtable

X

Any non aggregate expressions in the SELECT and GROUP BYclauses must be non-modified columns

X

DML allowed on detailed tables X X

Direct path data load allowed X X X

Materialized V

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