Oracle Materialized Views for
Replication
COUG Presentation, Feb 20, 2014
Jane Lamont, jlamont@geologic.com
Materialized Views 101
• Types and uses of materialized views
• Basic setup of materialized views
• Common types of refreshes used on
materialized views.
• Demo setup, refresh and drop
Bio
• 17 years experience working with Oracle databases in ON, NT,
and AB
• Certified OCP - 7, 8, 9i, 11g and E-Business Suite, 11g
• Instructor at SAIT, BA Program & DBA Fast-Track Program,
2001 - 2005
• Executive of COUG 2000 – 2008 and President 2003-2006
• Presented twice before at COUG
• Built and maintained > 3000 materialized views 5 to 25M
records in each in a warehousing environment at geoLOGIC
Systems for past 4 years
Data Warehouse Architecture
Views & Materialized Views
• Views are based on a query where the structure may/may not
be saved, and the results are cached only
• Materialized views are based on a query that is saved and
where the results reside in physical tables
• Snapshot is a previous term for materialized view. Will still
see reference to snapshots in Oracle docs
Materialized Views
• A materialized view (mview) is a replica of a master table from
a single point in time connected together via database links.
• Mviews are updated from one or more masters through
individual batch updates, called refreshes.
• Fast refresh is applying only changes to the master site to the
mview, enabled by a materialized view log that records the
changes to the master table.
• Complete refresh is a full copy of the master site to the
mview.
Materialized View Replication
Why Use Materialized Views?
• Ease Network Loads. Distribute the corporate database
amongst multiple sites, giving stable location for client
connection while staging area is updating/processing.
• Create a Mass Deployment Environment. Rollout db
infrastructure quickly and easily
• Enable Data Subsetting or Aggregation. Query of master
table(s). Query rewriting by the optimizer.
• Enable Disconnect Computing. No need for dedicated
network connection between databases
Materialized Views
•
Read-only
•
Updatable, must belong to group.
Types of Materialized Views
1. Primary Key – default. Based on the pk in the master.
2. Object – based on object table and created using the OF
type clause.
3. ROWID – based on the rowids in the master
4. Complex – if defining query does not meet restrictions to be
fast refreshed, such as CONNECT BY, INTERSECT, MINUS,
UNION ALL etc. can only be refreshed ‘complete’.
Materialized View Security
• Privileges
– CREATE MATERIALIZED VIEW
– CREATE TABLE
– SELECT object privilege on master table and its mview log,
if not using database link, otherwise is included in the link
Mview log in the Staging Database
• A master table on which the MView is based on has a MView log table
(MLOG$_ ) to hold the changed rows in the master table
• Analogous to the redo log.
• An entry in SYS.MLOG$ defines the MView log.
• A fast refresh is based on the rowids or primary keys
• Note: If the mview query is NOT simple then it cannot be fast refreshed so
it will NOT need a mview log.
• One master table/mview log can have > 1 mviews. Log ensures that all
mviews are refreshed and does not purge itself until all mviews are
refreshed.
• Also mviews may be refreshed from the same mview log at different times
so they are kept in synch by the timestamp of the fast refresh. 11gR2 now
uses commit SCN data instead of timestamps which improves the speed of
the mview refresh.
– SQL> create materialized log on emp with commit scn;
Mviews in the Warehouse
• A table in the mview site is referred to as the mview base
table.
• An unique index on the mview base table
• An entry in SYS.SNAP$ defining the mview.
• An entry in SYS.SLOG$ at master site.
• SQL>create materialized view emp_mv
refresh fast on demand as
select * from scott.emp@oracle.world;
• Note: if complete refresh, set the mview PCTFREE to 0 and
PCTUSED to 99 for maximum efficiency
DBMS_MVIEW supplied package
• Various procedures to design, build,
troubleshoot, repair mviews
– Refresh procedure
– Explain_mview procedure
– Purge_log procedure
– Register_mview procedure
Mview Refresh Procedure
• DBMS_MVIEW.REFRESH(‘<table_name>’, ‘COMPLETE’);
– Complete transfer of data from the master table to mview base table
- List of tables refreshed in single transaction, consistency across the
mviews to a single point in time and if 1 errors, none are refreshed,
add parameter: atomic_refresh=true
- DBMS_MVIEW.REFRESH(‘emp_mv, dept_mv’, ’complete’, atomic_refresh=true);
• DBMS_MVIEW.REFRESH(‘<table_name>’, ‘FAST’);
– Changes contained in the mview log are applied to the mview base
table.
Idea: Mview used for cut-over
• If time is limited to do a cut-over to a new database. Lots of
time to prep, no time to execute:
– RMAN cloning
– Standby database
– Mview drop with preserve table (includes indexes) clause
• SQL> drop materialized view on emp preserve table;
References
• Oracle Database Concepts, 11.2 Release
– E40540-01
• Oracle Database Advanced Replication, 11.2 Release
– E10706-06
• Oracle Database PL/SQL Packages and Types Reference, 11.2
Release
– E40758-03
My experiences
• Do not data pump mviews to new database, it loses its
registration in the master site, in the SYS.SLOG$.
• Logs can be ‘pesky’ and need to be rebuilt. Do complete
refreshes to all mviews first!
• Logs keep filling and never purging.
• Bug in 11gR2 where master table was in a 11gR2 version and
mview in 11gR1 version.
• Watch out for the database links if moving either master or
base table.
Demo
• 1 table replicated from OLTP or batch master table site in the
staging schema to a warehouse schema using a mview based
on the primary key of the master table. Then SQL SELECT
done against the warehouse
• Mview will be complete refreshed automatically when built,
then will be fast refreshed on demand as data received into
the staging database
• Mview Log will use commit scn
• Mview will be dropped preserving the table
Demo
Scott, data analyst,
connects to
warehouse
MView,
Sales_mv in WH
schema
Master table,
Sales and log in
HQ schema
Summary
• Consider using mviews in a situation where an
end user will need data refreshed periodically
Q&A