Materialized Views Presenter: Robert Webb Email: rwebb@RemoteControlDBA

Materialized Views

Presenter: Robert Webb

Email: [email protected]: (888) 235-8916Web: www.RemoteControlDBA.com

Materialized Views

Special kind of View to improve query execution times by precalculating Join and/or Aggregation operations prior to execution and storing the results in a database Table.

Similar to Indexes

Designed to increase query Execution Performance.

Transparent to SQL Applications allowing DBA’s to create and drop Materialized Views without affecting the validity of Applications.

Consume Storage Space.

Can be Partitioned.

Basic Terms

Dimension Table – Also known as a lookup table.

– Generally small.

– Describes the entities of a business, often in a categorical or hierarchical way.

– Examples: Location, Department, or Products.

Basic Terms (cont)

Fact Table – Also known as a Detail Table.

– Generally Very large.

– Records the Business Transactions.

– Examples: Sales and Expenses.

Creating a Materialized View

Init ParametersPrivilegesStatisticsMaterialized View Logs

(if using Fast Refresh)Materialized View DefinitionStatistics on Materialized View

Init Parameters

COMPATIBLE = 8.1.0 (or higher)

OPTIMIZER_MODE = Choose, All_Rows, or First_Rows

QUERY_REWRITE_ENABLED = TRUE

Privileges

GRANT QUERY REWRITE TO SCOTT;– If all base tables are owned by Scott.

GRANT GLOBAL QUERY REWRITE TO SCOTT;– If some base tables are not owned by Scott.

Note: All Privileges, including SELECT privilege on base tables, must be granted explicitly – not inherited from a ROLE.

Statistics

ANALYZE TABLES

DBMS_STATS

Materialized View Log

Record changes to Base Tables.

Required for Incremental (FAST) Refresh.

Syntax;

CREATE MATERIALIZED VIEW LOGSALES_LOG WITH ROWID(LOCATION, TIME, DOLLAR_SALES)INCLUDING NEW VALUES;

Materialized View DDL

Create Materialized View Example_mvPctfree 0 tablespace mviewsstorage (initial 128K next 128K pctincrease 0)ParallelBuild ImmediateRefresh Fast On CommitEnable Query Rewrite As Select ….

Materialized View DDL

Create Materialized View Example_mvPctfree 0 tablespace mviewsstorage (initial 128K next 128K pctincrease 0)ParallelBUILD IMMEDIATERefresh Fast On CommitEnable Query Rewrite As Select ….

Build Options

Build Immediate – Populates the Materialized View as it is Created.

Build Deferred – Create the Materialized View but does Not populate it with data.

Materialized View DDL

Create Materialized View Example_mvPctfree 0 tablespace mviewsstorage (initial 128K next 128K pctincrease 0)ParallelBuild ImmediateREFRESH FAST On CommitEnable Query Rewrite As Select ….

Refresh Options

Complete – Truncates existing data and recalculates the defining query.

Fast – Incrementally adds new data. The data is obtained from the Materialized View Log or Direct Path Log.

Force – Performs Fast Refresh if possible, otherwise a Complete Refresh is performed.

Never – Does not Refresh

Materialized View DDL

Create Materialized View Example_mvPctfree 0 tablespace mviewsstorage (initial 128K next 128K pctincrease 0)ParallelBuild ImmediateRefresh Fast ON COMMITEnable Query Rewrite As Select ….

Refresh Mode

ON COMMIT – Materialized View is refreshed automatically when transaction commits a change to an underlying table.– Restricted to Single Table Aggregate and Join

only Materialized Views.

ON DEMAND – User manually executes one of the refresh options available in the DBMS_MVIEW package.– DBMS_MVIEW.Refresh_All_Mviews.

Materialized View DDL

Create Materialized View Example_mv

Pctfree 0 tablespace mviews

storage (initial 128K next 128K pctincrease 0)

Parallel

Build Immediate

Refresh Fast On Commit

Enable Query Rewrite

As Select ….

Query Rewrite

Alerts the Optimizer that this Materialized View is eligible for Query Rewrite.

May be Enabled and Disabled.

Types of Materialized Views

Join only.

Single table aggregate.

Joins and aggregates.

Join Only

Allows expensive Joins to precalculated.

Fast Refresh Option available

On Commit Refresh Mode available.– Materialized View Log required for each table.– RowID’s must be in the Select list of the

Materialized View query.

For faster refresh create an index on the RowID column of the Materialized View.

Single Table Aggregate

Precalculate expensive Aggregates (Sum, Avg, Stddev, Count)

Fast Refresh Option available.– Requires Materialized View Log with the

INCLUDING NEW VALUES clause.

On Commit Refresh Mode available.

Count(*) is always required in Select clause.

Join and Aggregate

Precalculate expensive Joins and Aggregates.

Restricted Refresh options.

Fast Refresh Mode available– Only after Direct Path Loads.– Not available after DML (On Commit).

Query Rewrite

Query Rewrite is Transparent allowing Materialized Views to be added or dropped at any time without invalidating SQL or Applications.

Query Rewrite

When does Rewrite Occur

Query Rewrite must be enabled for the session.

Query Rewrite must be enabled for the Materialized View.

Rewrite Integrity Level allows use of Materialized View

All or Part of the results should be available from the Materialized View.

Enabling Query Rewrite

Globally with Init Parameter QUERY_REWRITE_ENABLED = TRUE

For Individual Views ALTER MATERIALIZED VIEW example_mv ENABLE QUERY REWRITE

Disabling Query Rewrite

Globally with Init Parameter QUERY_REWRITE_ENABLED = FALSE

For Individual ViewsALTER MATERIALIZED VIEW example_mv DISABLE QUERY REWRITE

Rewrite Integrity

Enforced – Default and Safest Integrity Level– Must contain fresh data– Relationships must be based on Enforced constraints.

Trusted– Data is Trusted to be accurate– Relationships declared in Dimensions & Rely

constraints are Trusted.

Stale Tolerated– Views with Stale data are Tolerated.– Useful for Point in Time snapshots.

Cost Based Rewrite

The Optimizer must be Cost Based to utilize Query Rewrite (Choose, First_Rows, All_Rows)

Statistics must be accurate for the underlying tables as well as the Materialized View.

Did the Rewrite Occur

Query execution times will often be all that is needed.

Explain Plans will show clearly that the Materialized View is being accessed rather than the underlying tables.

Summary Advisor

Materialized View Analysis and Advisory functions are available using the DBMS_OLAP Package.

– Recommend New Materialized Views.

– Report Actual Utilization of Materialized Views.

– Estimate Size of proposed Materialized Views.

Review – Materialized View

Special kind of View to improve query execution times by precalculating Join and/or Aggregation operations prior to execution and storing the results in a database Table.

Review – Similar to Indexes

Designed to increase query Execution Performance.

Transparent to SQL Applications allowing DBA’s to create and drop Materialized Views without affecting the validity of Applications.

Consume Storage Space.

Can be Partitioned.

Review – Creating Materialized Views

Init ParametersPrivilegesStatisticsMaterialized View Logs

(if using Fast Refresh)Materialized View DefinitionStatistics on Materialized View