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11 Copyright © 2005, Oracle. All rights reserved. Refreshing Materialized Views.

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Presentation on theme: "11 Copyright © 2005, Oracle. All rights reserved. Refreshing Materialized Views."— Presentation transcript:

1 11 Copyright © 2005, Oracle. All rights reserved. Refreshing Materialized Views

2 11-2 Copyright © 2005, Oracle. All rights reserved. Objectives After completing this lesson, you should be able to do the following: Describe the various refresh methods and modes Refresh materialized views Create fast-refreshable materialized views Explain why materialized views cannot be fast refreshable Create materialized view logs Describe the benefit of Partition Change Tracking Use parallel refreshes

3 11-3 Copyright © 2005, Oracle. All rights reserved. Refresh Options Refresh options specified when the materialized view is created: COMPLETE : Re-executes query FAST : Refresh is based on delta –Using materialized view logs –Using direct loader log: ALL_SUMDELTA –Using Partition Change Tracking refresh FORCE : FAST if possible, otherwise COMPLETE NEVER

4 11-4 Copyright © 2005, Oracle. All rights reserved. Full Notes Page

5 11-5 Copyright © 2005, Oracle. All rights reserved. Refresh Modes Refresh execution modes specified when the materialized view is created: ON DEMAND : Manual refresh using procedures in DBMS_MVIEW package (default) ON COMMIT : Refresh performed at transaction commit –Only possible for fast-refreshable materialized views – ON COMMIT REFRESH object privilege needed –In case of failure, subsequent refreshes are manual

6 11-6 Copyright © 2005, Oracle. All rights reserved. Refresh at Scheduled Time Schedule refresh at materialized view creation time with START WITH and NEXT : CREATE MATERIALIZED VIEW emp_data_mv … REFRESH FAST NEXT sysdate + 7 … CREATE MATERIALIZED VIEW cust_data_mv … REFRESH START WITH ROUND(SYSDATE + 1) + 11/24 NEXT NEXT_DAY(TRUNC(SYSDATE),'MONDAY')+ 15/24 …

7 11-7 Copyright © 2005, Oracle. All rights reserved. Manual Refresh Using DBMS_MVIEW For ON DEMAND refresh Three procedures in the DBMS_MVIEW package: – REFRESH – REFRESH_ALL_MVIEWS – REFRESH_DEPENDENT The refresh can be performed by: –Foreground session, if no job queue process is started –One or multiple job queue processes, if JOB_QUEUE_PROCESSES is set –Multiple refresh, which can be done in parallel

8 11-8 Copyright © 2005, Oracle. All rights reserved. Full Notes Page

9 11-9 Copyright © 2005, Oracle. All rights reserved. Refresh Specific Materialized Views with REFRESH exec DBMS_MVIEW.REFRESH(- 'MV1,MV2','FC','',FALSE,TRUE,0,0,0,FALSE); List of materialized views Refresh method for each Do not push changes Continue when there are errors Refresh each in a separate transaction

10 11-10 Copyright © 2005, Oracle. All rights reserved. Full Notes Page

11 11-11 Copyright © 2005, Oracle. All rights reserved. Refresh All Materialized Views with REFRESH_ALL_MVIEWS VARIABLE fail NUMBER; exec DBMS_MVIEW.REFRESH_ALL_MVIEWS(- :fail, 'C','', TRUE, FALSE); Returns number of failed refreshes Refresh method for each Continue when there are errors Refresh each in a separate transaction

12 11-12 Copyright © 2005, Oracle. All rights reserved. Refresh Dependent Materialized Views with REFRESH_DEPENDENT Refresh all materialized views directly depending on a list of detail tables or materialized views: VARIABLE fail NUMBER; exec DBMS_MVIEW.REFRESH_DEPENDENT(- :fail,'CUSTOMERS,SALES','CF','',TRUE, FALSE); List of materialized views Refresh method for each Continue when there are errors Refresh each in a separate transaction

13 11-13 Copyright © 2005, Oracle. All rights reserved. Finding Dependent Materialized Views Find direct dependencies only. Can be used to determine the materialized view refresh order for nested materialized views VARIABLE deplist VARCHAR2(500) exec DBMS_MVIEW.GET_MV_DEPENDENCIES(- 'CUSTOMERS,SALES',:deplist); List of materialized views Returns list of dependent materialized views

14 11-14 Copyright © 2005, Oracle. All rights reserved. When Does a Complete Refresh Occur? Initial build of the materialized view ( BUILD IMMEDIATE ) First refresh for BUILD DEFERRED materialized views Manual request

15 11-15 Copyright © 2005, Oracle. All rights reserved. Conditions That Effect Possibility of Fast Refresh Existence of materialized view logs Materialized views detail query Types of modifications made to the detail tables Sequence of modifications made to the detail tables

16 11-16 Copyright © 2005, Oracle. All rights reserved. Explain Materialized View DBMS_MVIEW.EXPLAIN_MVIEW accepts: –Materialized view name, or –SQL statement Advises what is possible for: –An existing materialized view –A potential materialized view before you create it Results are stored in MV_CAPABILITIES_TABLE (relational table) or in a VARRAY. utlxmv.sql must be executed as the current user to create MV_CAPABILITIES_TABLE.

17 11-17 Copyright © 2005, Oracle. All rights reserved. Using DBMS_MVIEW.EXPLAIN_MVIEW EXEC dbms_mview.explain_mview('sales_sum', '123'); SELECT capability_name, possible, related_text, msgtxt FROM mv_capabilities_table WHERE statement_id = '123' ORDER BY seq; CAPABILITY_NAME P REL_TEXT MSGTXT ---------------- - -------- --------------------- … REFRESH_COMPLETE Y REFRESH_FAST N REFRESH_FAST_AFTER_INSERT N TIMES mv log must have new values …

18 11-18 Copyright © 2005, Oracle. All rights reserved. Materialized View Logs: General Concepts Definition: Table that contains rows describing changes made to a detail table The logs are required if fast refresh is used. Log definition must include the WITH ROWID clause. For joins and aggregate materialized views to be fast-refreshable after INSERT s, logs must include: –Every column referenced in the materialized view –The INCLUDING NEW VALUES clause An INSERT, UPDATE, DELETE combination on multiple detail tables requires the log to include the SEQUENCE clause. SEQUENCE provides ordering information.

19 11-19 Copyright © 2005, Oracle. All rights reserved. Creating a Materialized View Log CREATE MATERIALIZED VIEW LOG ON sh.sales TABLESPACE users NOLOGGING PARALLEL WITH SEQUENCE, ROWID (quantity_sold,amount_sold) INCLUDING NEW VALUES;

20 11-20 Copyright © 2005, Oracle. All rights reserved. Restrictions for Fast-Refreshable Materialized Views General restrictions: –No non-repeating expressions –No references to RAW or LONG RAW data types –Support for subqueries in the WHERE clause limited to data subsetting ( WHERE EXISTS ) Additional restrictions apply for specific types of materialized views.

21 11-21 Copyright © 2005, Oracle. All rights reserved. Indexes and Fast-Refreshable Materialized Views Fast refresh joins and aggregate materialized view –The Oracle server automatically creates a unique functional index on a materialized views keys for better fast refresh performance. –Index expression: SYS_OP_MAP_NONNULL(keyn) – USING [NO] INDEX clause at creation time Fast refresh joins-only materialized view –No index is automatically created. –For better fast refresh performance, create an index on row ID columns of the select list.

22 11-22 Copyright © 2005, Oracle. All rights reserved. Specifying REFRESH FORCE Materialized view may be fast refreshable in some, but not all cases. –Joins-only materialized view without row IDs from one of its detail tables can still be fast refreshed whenever updating only the other detail tables. –Should be exception and should not be generalized Use REFRESH FORCE option during creation to ensure fast refresh whenever possible. Query DBA_MVIEWS.FAST_REFRESHABLE to determine whether the materialized view is eligible for fast refresh.

23 11-23 Copyright © 2005, Oracle. All rights reserved. Full Notes Page

24 11-24 Copyright © 2005, Oracle. All rights reserved. Partition Change Tracking (PCT) Refresh Fast refresh uses materialized view log or direct- loader log. Certain cases prevent using a materialized view log thus preventing general fast refresh. –Dropping of a detail tables partition –Mixed DML when the materialized view uses the MIN function Maintain staleness at partition level: –Only detected for partitioned detail tables –Only specific sections of the materialized view need to be refreshed –Rows corresponding to impacted partitions only –Should be faster than a complete refresh

25 11-25 Copyright © 2005, Oracle. All rights reserved. PCT Refresh Requirements One of the materialized view detail tables must be partitioned: RANGE, COMPOSITE, or LIST. Partition key must consist of a single column. SELECT and GROUP BY list must contain: partition key column, partition marker ( DBMS_MVIEW.PMARKER ), ROWID, or join-dependent expression of one of its detail tables. No reference to views or outer joins

26 11-26 Copyright © 2005, Oracle. All rights reserved. When Is PCT Refresh Used? Fast refresh is requested, and A detail table is partitioned, and PCT refresh is possible for the materialized view, and One or both of the following two conditions have occurred on detail tables: –Partition DDL operations –Combinations of DML/direct-loads not handled by fast refresh Requested using P parameter with refresh procedures FAST_PCT is indicated in LAST_REFRESH_TYPE.

27 11-27 Copyright © 2005, Oracle. All rights reserved. Partition Key or Partition Marker? Cardinality = #part_keys x distinct(prod_name) CREATE MATERIALIZED VIEW cost_product_mv (tid, pname, su, cu, cnt) BUILD IMMEDIATE REFRESH FAST ON DEMAND ENABLE QUERY REWRITE AS SELECT time_id, prod_name, SUM(unit_cost), COUNT(unit_cost), COUNT(*) FROM sh.costs c, sh.products p WHERE c.prod_id = p.prod_id GROUP BY time_id, prod_name;

28 11-28 Copyright © 2005, Oracle. All rights reserved. Partition Key or Partition Marker? Cardinality = #partitions x distinct(prod_name) CREATE MATERIALIZED VIEW cost_product_pm_mv (pm, pname, su, cu, cnt) BUILD IMMEDIATE REFRESH FAST ON DEMAND ENABLE QUERY REWRITE AS SELECT DBMS_MVIEW.PMARKER(c.rowid), prod_name, SUM(unit_cost), COUNT(unit_cost), COUNT(*) FROM costs c, products p WHERE c.prod_id = p.prod_id GROUP BY DBMS_MVIEW.PMARKER(c.rowid), prod_name;

29 11-29 Copyright © 2005, Oracle. All rights reserved. PCT Using ROWID CREATE MATERIALIZED VIEW pct_rowid_mv (srid, prid, cid, pname) BUILD IMMEDIATE REFRESH FORCE ENABLE QUERY REWRITE AS SELECT s.rowid, p.rowid, s.channel_id, p.prod_name FROM sales s, products p WHERE s.prod_id = p.prod_id;

30 11-30 Copyright © 2005, Oracle. All rights reserved. Consider Fresh: Fastest Refresh Scenario FactMV Drop partition ALTER MATERIALIZED VIEW CONSIDER FRESH 1 2 3

31 11-31 Copyright © 2005, Oracle. All rights reserved. CONSIDER FRESH Refresh Implications STALENESS column of DBA_MVIEWS set to UNKNOWN until next complete refresh Until next complete refresh, PCT refresh no longer possible Fast refresh still possible

32 11-32 Copyright © 2005, Oracle. All rights reserved. General Tips for Materialized View Refresh For complete refresh using DBMS_MVIEW interface, set ATOMIC_REFRESH to FALSE. Disable materialized view logging before refreshing ( NOLOGGING ). Always check alert.log and trace files after a refresh. Try to perform only one type of change before refreshing. Try to refresh directly after one table load. For ON COMMIT refresh, try to group DMLs in one single transaction or use short transactions.

33 11-33 Copyright © 2005, Oracle. All rights reserved. Refresh Tips for Nested Materialized Views For ON DEMAND refresh: –You must refresh in the correct order. –Refresh is based on detail materialized views independently of their STALENESS. For ON COMMIT refresh (simple joins or single- table aggregate): –Appropriate refresh order is automatically used. –In case of problem, you must refresh manually.

34 11-34 Copyright © 2005, Oracle. All rights reserved. Summary In this lesson, you should have learned how to: Create materialized views with different refresh methods and modes Use the DBMS_MVIEW package to refresh materialized views Use the DBMS_MVIEW package to explain materialized views Create materialized view logs Create materialized views supporting PCT

35 11-35 Copyright © 2005, Oracle. All rights reserved. Practice 11: Overview This practice covers the following topics: Using various refresh types: – COMPLETE – FAST – FAST_PCT Explaining materialized view refresh capabilities Creating materialized view logs

36 11-36 Copyright © 2005, Oracle. All rights reserved.


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