Release 1 (9.0.1)
Part Number A90237-01
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| Oracle9i Data Warehousing Guide Release 1 (9.0.1) Part Number A90237-01 |
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This chapter discusses how to load and refresh a data warehouse, and discusses:
ETL (Extraction, Transformation and Loading) is done on a scheduled basis to reflect changes made to the original source system. During this step, you physically insert the new, clean data into the production data warehouse schema, and take all of the other steps necessary (such as building indexes, validating constraints, taking backups) to make this new data available to the end users. Once all of this data has been loaded into the data warehouse, the materialized views have to be updated to reflect the latest data.
The partitioning scheme of the data warehouse is often crucial in determining the efficiency of refresh operations in the data warehouse load process. In fact, the load process is often the primary consideration in choosing the partitioning scheme of data warehouse tables and indexes.
The partitioning scheme of the largest data warehouse tables (for example, the fact table in a star schema) should be based upon the loading paradigm of the data warehouse.
Most data warehouses are loaded with new data on a regular schedule. For example, every night, week, or month, new data is brought into the data warehouse. The data being loaded at the end of the week or month typically corresponds to the transactions for the week or month. In this very common scenario, the data warehouse is being loaded by time. This suggests that the data warehouse tables should be partitioned on a date column. In our data warehouse example, suppose the new data is loaded into the sales table every month. Furthermore, the sales table has been partitioned by month. These steps show how the load process will proceed to add the data for a new month (January 2001) to the table sales:
sales_01_2001. This data can be directly loaded into sales_01_2001 from outside the data warehouse, or this data can be the result of previous data transformation operations that have already occurred in the data warehouse. sales_01_2001 has the exact same columns, datatypes, and so forth, as the sales table. Gather statistics on the sales_01_2001 table.
sales_01_2001. Again, the indexes and constraints on sales_01_2001 should be identical to the indexes and constraints on sales. Indexes can be built in parallel and should use the NOLOGGING and the COMPUTE STATISTICS options. For example:
CREATE BITMAP INDEX sales_01_2001_customer_id_bix ON sales_01_2001(customer_id) TABLESPACE sales_idx NOLOGGING PARALLEL 8 COMPUTE STATISTICS;
Apply all constraints to the sales_01_2001 table that are present on the sales table. This includes referential integrity constraints. A typical constraint would be:
ALTER TABLE sales_01_2001 ADD CONSTRAINT sales_customer_id REFERENCES customer(customer_id) ENABLE NOVALIDATE;
If the partitioned table sales has a primary or unique key that is enforced with a global index structure, please ensure that the constraint on sales_jan01 is validated without the creation of an index structure, like:
ALTER TABLE sales_01_2001 ADD CONSTRAINT sales_pk_jan01 PRIMARY KEY (sales_transaction_id) DISABLE VALIDATE;
The creation of the constraint with ENABLE clause would cause the creation of a unique index, which does not match a local index structure of the partitioned table. The exchange command would fail.
sales_01_2001 table to the sales table.
In order to add this new data to the sales table, we need to do two things. First, we need to add a new partition to the sales table. We will use the ALTER TABLE ... ADD PARTITION statement. This will add an empty partition to the sales table:
ALTER TABLE sales ADD PARTITION sales_01_2001 VALUES LESS THAN (TO_DATE('01-FEB-2001', 'DD-MON-YYYY'));
Then, we can add our newly created table to this partition using the EXCHANGE PARTITION operation. This will exchange the new, empty partition with the newly loaded table.
ALTER TABLE sales EXCHANGE PARTITION sales_01_2001 WITH TABLE sales_01_2001 INCLUDING INDEXES WITHOUT VALIDATION UPDATE GLOBAL INDEXES;
The EXCHANGE operation will preserve the indexes and constraints that were already present on the sales_01_2001 table. For unique constraints (such as the unique constraint on sales_transaction_id), you can use the UPDATE GLOBAL INDEXES clause, as shown above. This will automatically maintain your global index structures as part of the partition maintenance operation and keep them accessible throughout the whole process. If there were only foreign-key constraints, the exchange operation would be instantaneous.
The benefits of this partitioning technique are significant. First, the new data is loaded with minimal resource utilization. The new data is loaded into an entirely separate table, and the index processing and constraint processing are applied only to the new partition. If the sales table was 50 GB and had 12 partitions, then a new month's worth of data contains approximately 4 GB. Only the new month's worth of data needs to be indexed. None of the indexes on the remaining 46 GB of data needs to be modified at all. This partitioning scheme additionally ensures that the load processing time is directly proportional to the amount of new data being loaded, not to the total size of the sales table.
Second, the new data is loaded with minimal impact on concurrent queries. All of the operations associated with data loading are occurring on a separate sales_01_2001 table. Therefore, none of the existing data or indexes of the sales table is affected during this data refresh process. The sales table and its indexes remain entirely untouched throughout this refresh process.
Third, in case of the existence of any global indexes, those are incrementally maintained as part of the exchange command. This maintenance does not affect the availability of the existing global index structures.
The exchange operation can be viewed as a publishing mechanism. Until the data warehouse administrator exchanges the sales_01_2001 table into the sales table, end users cannot see the new data. Once the exchange has occurred, then any end user query accessing the sales table will immediately be able to see the sales_01_2001 data.
Partitioning is useful not only for adding new data but also for removing data. Many data warehouses maintain a rolling window of data. For example, the data warehouse stores the most recent 36 months of sales data. Just as a new partition can be added to the sales table (as described above), an old partition can be quickly (and independently) removed from the sales table. The above two benefits (reduced resources utilization and minimal end-user impact) are just as pertinent to removing a partition as they are to adding a partition.
This example is a simplification of the data warehouse load scenario. Real-world data warehouse refresh characteristics are always more complex. However, the advantages of this rolling window approach are not diminished in more complex scenarios.
Consider two typical scenarios:
Solution: Partition by week or month (as appropriate). Use INSERT to add the new data to an existing partition. The INSERT operation only affects a single partition, so the benefits described above remain intact. The INSERT operation could occur while the partition remains a part of the table. Inserts into a single partition can be parallelized:
INSERT INTO sales PARTITION (sales_01_2001) SELECT * FROM new_sales;
The indexes of this sales partition will be maintained in parallel as well. An alternative is to use the EXCHANGE operation. You can do this by exchanging the sales_01_2001 partition of the sales table and then using an INSERT operation. You might prefer this technique when dropping and rebuilding indexes is more efficient than maintaining them.
Solution 1: Use parallel SQL operations (such as CREATE TABLE ... AS SELECT) to separate the new data from the data in previous time periods. Process the old data separately using other techniques.
New data feeds are not solely time based. You can also feed new data into a data warehouse with data from multiple operational systems on a business need basis. For example, the sales data from direct channels may come into the data warehouse separately from the data from indirect channels. For business reasons, it may furthermore make sense to keep the direct and indirect data in separate partitions.
Solution 2: Oracle supports concatenated partitioning keys. The sales table could be partitioned by month and channel. Care must be taken with this approach to ensure that the partition pruning techniques (when querying the sales table) are understood prior to implementation.
Another possibility is composite (range/hash) partitioning. This approach is feasible only if the second key has a high cardinality. In this example, channel has only two possible values, so that it would not be a good candidate for a hash-partitioning key.
You can optimize DML performance through the techniques listed in this section.
Commonly, the data that is extracted from a source system is not simply a list of new records that needs to be inserted into the data warehouse. Instead, this new data set is a combination of new records as well as modified records. For example, suppose that most of data extracted from the OLTP systems will be new sales transactions. These records will be inserted into the warehouse's sales table, but some records may reflect modifications of previous transactions, such as returned merchandise or transactions that were incomplete or incorrect when initially loaded into the data warehouse. These records require updates to the sales table.
As a typical scenario, suppose that there is a table called new_sales that contains both inserts and updates that will be applied to the sales table. When designing the entire data warehouse load process, it was determined that the new_sales table would contain records with the following semantics:
sales_transaction_id of a record in new_sales already exists in sales, then update the sales table by adding the sales_dollar_amount and sales_quantity_sold values from the new_sales table to the existing row in the sales table.
new_sales table into the sales table.
This UPDATE-ELSE-INSERT operation is often called an upsert or merge. A merge can be executed using one SQL statement in Oracle9i, though it required two earlier.
The first SQL statement updates the appropriate rows in the sales tables, while the second SQL statement inserts the rows:
UPDATE (SELECT s.sales_quantity_sold AS s_quantity, s.sales_dollar_amount AS s_dollar, n.sales_quantity_sold AS n_quantity, n.sales_dollar_amount AS n_dollar FROM sales s, new_sales n WHERE s.sales_transaction_id = n.sales_transaction_id) sales_view SET s_quantity = s_quantity + n_quantity, s_dollar = s_dollar + n_dollar; INSERT INTO sales SELECT * FROM new_sales s WHERE NOT EXISTS (SELECT 'x' FROM FROM sales t WHERE s.sales_transaction_id = t.sales_transaction_id);
The new, faster way of upserting data is shown below.
MERGE INTO sales s USING new_sales n ON (s.sales_transaction_id = n.sales_transaction_id) WHEN MATCHED THEN UPDATE s_quantity = s_quantity + n_quantity, s_dollar = s_dollar + n_dollar WHEN NOT MATCHED THEN INSERT (sales_quantity_sold, sales_dollar_amount) VALUES (n.sales_quantity_sold, n.sales_dollar_amount);
An alternative implementation of upserts is to utilize a PL/SQL package, which successively reads each row of the new_sales table and applies if-then logic to either update or insert the new row into the sales table. A PL/SQL-based implementation is effective when the new_sales table is small, although the SQL approach will often be more efficient for larger data volumes.
In some data warehousing environments, you might want to insert new data into tables in order to guarantee referential integrity. For example, a data warehouse may derive sales from an operational system that retrieves data directly from cash registers. sales is refreshed nightly. However, the data for the product dimension table may be derived from a separate operational system. The product dimension table may only be refreshed once per week, because the product table changes relatively slowly. If a new product was introduced on Monday, then it is possible for that product's product_id to appear in the sales data of the data warehouse before that product_id has been inserted into the data warehouses product table.
Although the sales transactions of the new product may be valid, this sales data will not satisfy the referential integrity constraint between the product dimension table and the sales fact table. Rather than disallow the new sales transactions, you might choose to insert the sales transactions into the sales table.
However, you might also wish to maintain the referential integrity relationship between the sales and product tables. This can be accomplished by inserting new rows into the product table as placeholders for the unknown products.
As in previous examples, we assume that the new data for the sales table will be staged in a separate table, new_sales. Using a single INSERT statement (which can be parallelized), the product table can be altered to reflect the new products:
INSERT INTO PRODUCT_ID (SELECT sales_product_id, 'Unknown Product Name', NULL, NULL ... FROM new_sales WHERE sales_product_id NOT IN (SELECT product_id FROM product));
Occasionally, it is necessary to remove large amounts of data from a data warehouse. A very common scenario is the rolling window discussed previously, in which older data is rolled out of the data warehouse to make room for new data.
However, sometimes other data might need to be removed from a data warehouse. Suppose that a retail company has previously sold products from MS Software, and that MS Software has subsequently gone out of business. The business users of the warehouse may decide that they are no longer interested in seeing any data related to MS Software, so this data should be deleted.
One approach to removing a large volume of data is to use parallel delete:
DELETE FROM sales WHERE sales_product_id IN (SELECT product_id FROM product WHERE product_category = 'MS Software');
This SQL statement will spawn one parallel process per partition. This approach will be much more efficient than a serial DELETE statement, and none of the data in the sales table will need to be moved.
However, this approach also has some disadvantages. When removing a large percentage of rows, the DELETE statement will leave many empty row-slots in the existing partitions. If new data is being loaded using a rolling window technique (or is being loaded using direct-path INSERT or load), then this storage space will not be reclaimed. Moreover, even though the DELETE statement is parallelized, there might be more efficient methods. An alternative method is to re-create the entire sales table, keeping the data for all product categories except MS Software.
CREATE TABLE sales2 AS SELECT * FROM sales, product WHERE sales.sales_product_id = product.product_id AND product_category <> 'MS Software' NOLOGGING PARALLEL (DEGREE 8) #PARTITION ... ; #create indexes, constraints, and so on DROP TABLE SALES; RENAME SALES2 TO SALES;
This approach may be more efficient than a parallel delete. However, it is also costly in terms of the amount of disk space, because the sales table must effectively be instantiated twice.
An alternative method to utilize less space is to re-create the sales table one partition at a time:
CREATE TABLE sales_temp AS SELECT * FROM sales WHERE 1=0; INSERT INTO sales_temp PARTITION (sales_99jan) SELECT * FROM sales, product WHERE sales.sales_product_id = product.product_id AND product_category <> 'MS Software'; <create appropriate indexes and constraints on sales_temp> ALTER TABLE sales EXCHANGE PARTITION sales_99jan WITH TABLE sales_temp;
Continue this process for each partition in the sales table.
When creating a materialized view, you have the option of specifying whether the refresh occurs ON DEMAND or ON COMMIT. In the case of ON COMMIT, the materialized view is changed every time a transaction commits, which changes data used by the materialized view, thus ensuring that the materialized view always contains the latest data. Alternatively, you can control the time when refresh of the materialized views occurs by specifying ON DEMAND. In this case, the materialized view can only be refreshed by calling one of the procedures in the DBMS_MVIEW package.
DBMS_MVIEW provides three different types of refresh operations.
DBMS_MVIEW.REFRESH
Refresh one or more materialized views.
DBMS_MVIEW.REFRESH_ALL_MVIEWS
Refresh all materialized views.
DBMS_MVIEW.REFRESH_DEPENDENT
Refresh all table-based materialized views that depend on a specified detail table or list of detail tables.
Performing a refresh operation requires temporary space to rebuild the indexes and can require additional space for performing the refresh operation itself. Some sites might prefer not to refresh all of their materialized views at the same time: as soon as some underlying detail data has been updated, all materialized views using this data will become stale. Therefore, if you defer refreshing your materialized views, you can either rely on your chosen rewrite integrity level whether or not a stale materialized view can be used for query rewrite, or you can temporarily disable query rewrite with an ALTER SYSTEM SET QUERY_REWRITE_ENABLED = FALSE statement. After refreshing the materialized views, you can reenable query rewrite as the default for all sessions in the current database instance by specifying ALTER SYSTEM SET QUERY_REWRITE_ENABLED as TRUE. Refreshing a materialized view automatically updates all of its indexes. In the case of full refresh, this requires temporary sort space to rebuild all indexes during refresh. This is because the full refresh truncates or deletes the table before inserting the new full data volume. If insufficient temporary space is available to rebuild the indexes, then you must explicitly drop each index or mark it UNUSABLE prior to performing the refresh operation.
If you anticipate performing insert, update or delete operations on tables referenced by a materialized view concurrently with the refresh of that materialized view, and that materialized view includes joins and aggregation, Oracle recommends you use ON COMMIT fast refresh rather than ON DEMAND fast refresh.
A complete refresh occurs when the materialized view is initially defined as BUILD IMMEDIATE, unless the materialized view references a prebuilt table. For materialized views using BUILD DEFERRED, a complete refresh must be requested before it can be used for the first time. A complete refresh may be requested at any time during the life of any materialized view. The refresh involves reading the detail tables to compute the results for the materialized view. This can be a very time-consuming process, especially if there are huge amounts of data to be read and processed. Therefore, you should always consider the time required to process a complete refresh before requesting it.
However, there are cases when the only refresh method available for an already built materialized view is complete refresh because the materialized view does not satisfy the conditions specified in the following section for a fast refresh.
Most data warehouses have periodic incremental updates to their detail data. As described in "Schema Design Guidelines for Materialized Views", you can use the SQL*Loader or any bulk load utility to perform incremental loads of detail data. Fast refresh of your materialized views is usually efficient, because instead of having to recompute the entire materialized view, the changes are applied to the existing data. Thus, processing only the changes can result in a very fast refresh time.
A materialized view can be refreshed automatically using the ON COMMIT method. Therefore, whenever a transaction commits which has updated the tables on which a materialized view is defined, those changes will be automatically reflected in the materialized view. The advantage of using this approach is you never have to remember to refresh the materialized view. The only disadvantage is the time required to complete the commit will be slightly longer because of the extra processing involved. However, in a data warehouse, this should not be an issue because there is unlikely to be concurrent processes trying to update the same table.
When a materialized view is refreshed ON DEMAND, one of three refresh methods can be specified as shown in the following table. You can define a default option during the creation of the materialized view.
Three refresh procedures are available in the DBMS_MVIEW package for performing ON DEMAND refresh. Each has its own unique set of parameters.
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See Also:
Oracle9i Supplied PL/SQL Packages and Types Reference for detailed information about the |
Use the DBMS_MVIEW.REFRESH procedure to refresh one or more materialized views. Some parameters are used only for replication, so they are not mentioned here. The required parameters to use this procedure are:
TRUE or FALSE)
A Boolean parameter. If set to TRUE, the number_of_failures output parameter will be set to the number of refreshes that failed, and a generic error message will indicate that failures occurred. The alert log for the instance will give details of refresh errors. If set to FALSE, the default, then refresh will stop after it encounters the first error, and any remaining materialized views in the list will not be refreshed.
FALSE, 0,0,0.
TRUE or FALSE)
If set to TRUE, then all refreshes are done in one transaction. If set to FALSE, then the refresh of each specified materialized view is done in a separate transaction.
For example, to perform a fast refresh on the materialized view cal_month_sales_mv, the DBMS_MVIEW package would be called as follows:
DBMS_MVIEW.REFRESH('CAL_MONTH_SALES_MV', 'F', '', TRUE, FALSE, 0,0,0, FALSE);
Multiple materialized views can be refreshed at the same time, and they do not all have to use the same refresh method. To give them different refresh methods, specify multiple method codes in the same order as the list of materialized views (without commas). For example, the following specifies that cal_month_sales_mv be completely refreshed and fweek_pscat_sales_mv receive a fast refresh.
DBMS_MVIEW.REFRESH('CAL_MONTH_SALES_MV, FWEEK_PSCAT_SALES_MV', 'CF', '', TRUE, FALSE, 0,0,0, FALSE);
If the refresh method is not specified, the default refresh method as specified in the materialized view definition will be used.
An alternative to specifying the materialized views to refresh is to use the procedure DBMS_MVIEW.REFRESH_ALL_MVIEWS. This procedure refreshes all materialized views. If any of the materialized views fails to refresh, then the number of failures is reported.
The parameters for this procedure are:
OUT variable)
TRUE or FALSE)
A Boolean parameter. If set to TRUE, the number_of_failures output parameter will be set to the number of refreshes that failed, and a generic error message will indicate that failures occurred. The alert log for the instance will give details of refresh errors. If set to FALSE, the default, then refresh will stop after it encounters the first error, and any remaining materialized views in the list will not be refreshed.
TRUE or FALSE)
If set to TRUE, then all refreshes are done in one transaction. If set to FALSE, then the refresh of each specified materialized view is done in a separate transaction.
An example of refreshing all materialized views is:
DBMS_MVIEW.REFRESH_ALL_MVIEWS(failures,'C','',FALSE,FALSE);
The third procedure, DBMS_MVIEW.REFRESH_DEPENDENT, refreshes only those materialized views that depend on a specific table or list of tables. For example, suppose the changes have been received for the orders table but not for customer payments. The refresh dependent procedure can be called to refresh only those materialized views that reference the orders table.
The parameters for this procedure are:
OUT variable)
TRUE or FALSE)
A Boolean parameter. If set to TRUE, the number_of_failures output parameter will be set to the number of refreshes that failed, and a generic error message will indicate that failures occurred. The alert log for the instance will give details of refresh errors. If set to FALSE, the default, then refresh will stop after it encounters the first error, and any remaining materialized views in the list will not be refreshed.
TRUE or FALSE)
If set to TRUE, then all refreshes are done in one transaction. If set to FALSE, then the refresh of each specified materialized view is done in a separate transaction.
In order to perform a full refresh on all materialized views that reference the customers table, specify:
DBMS_MVIEW.REFRESH_DEPENDENT(failures, 'CUSTOMERS', 'C', '', FALSE, FALSE );
To obtain the list of materialized views that are directly dependent on a given object (table or materialized view), use the procedure DBMS_MVIEW.GET_MV_DEPENDENCIES to determine the dependent materialized views for a given table, or for deciding the order to refresh nested materialized views.
DBMS_MVIEW.GET_MV_DEPENDENCIES(mvlist IN VARCHAR2, deplist OUT VARCHAR2)
The input to the above function is the name or names of the materialized view. The output is a comma separated list of the materialized views that are defined on it. For example:
GET_MV_DEPENDENCIES("JOHN.SALES_REG, SCOTT.PROD_TIME", deplist)
populates deplist with the list of materialized views defined on the input arguments. For example:
deplist <= "JOHN.SUM_SALES_WEST, JOHN.SUM_SALES_EAST, SCOTT.SUM_PROD_MONTH".
Job queues can be used to refresh multiple materialized views in parallel. If queues are not available, fast refresh will sequentially refresh each view in the foreground process. The order in which the materialized views are refreshed cannot be guaranteed. To make queues available, you must set the JOB_QUEUE_PROCESSES parameter. This parameter defines the number of background job queue processes and determines how many materialized views can be refreshed concurrently. This parameter is only effective when atomic_refresh is set to FALSE.
If the process that is executing DBMS_MVIEW.REFRESH is interrupted or the instance is shut down, any refresh jobs that were executing in job queue processes will be requeued and will continue running. To remove these jobs, use the DBMS_JOB.REMOVE procedure.
Not all materialized views may be fast refreshable. Therefore, use the package DBMS_MVIEW.EXPLAIN_MVIEW to determine what refresh methods are available for a materialized view.
Set the following parameters:
PARALLEL_MAX_SERVERS should be set high enough to take care of parallelism. You need to consider the number of slaves needed for the refresh statement. For example, with a DOP of eight, you need 16 slave processes.
PGA_AGGREGATE_TARGET should be set for the instance to manage the memory usage for sorts and joins automatically. If the memory parameters are set manually, SORT_AREA_SIZE should be less than HASH_AREA_SIZE.
OPTIMIZER_MODE should equal ALL_ROWS (cost-based optimization).
Remember to analyze all tables and indexes for better cost-based optimization.
While a job is running, you can query the V$SESSION_LONGOPS view to tell you the progress of each materialized view being refreshed.
SELECT * FROM V$SESSION_LONGOPS;
To look at the progress of which jobs are on which queue, use:
SELECT * FROM DBA_JOBS_RUNNING;
Three views are provided for checking the status of a materialized view:
To check if a materialized view is fresh or stale, issue the following statement:
SELECT MVIEW_NAME, STALENESS, LAST_REFRESH_TYPE, COMPILE_STATE FROM USER_MVIEWS ORDER BY MVIEW_NAME; MVIEW_NAME STALENESS LAST_REF COMPILE_STATE ---------- --------- -------- ------------- CUST_MTH_SALES_MV FRESH FAST NEEDS_COMPILE PROD_YR_SALES_MV FRESH FAST VALID
If the compile_state column shows NEEDS COMPILE, the other displayed column values cannot be trusted as reflecting the true status. Use
ALTER MATERIALIZED VIEW [materialized_view_name] COMPILE;
to revalidate the materialized view and then reissue the SELECT statement.
Following are some guidelines for using the refresh mechanism for materialized views with aggregates.
ROWID, SEQUENCE and INCLUDING NEW VALUES clauses.
Include all columns from the table likely to be used in materialized views in the materialized view logs.
Fast refresh may be possible even if the SEQUENCE option is omitted from the materialized view log. If it can be determined that only inserts or deletes will occur on all the detail tables, then the materialized view log does not require the SEQUENCE clause. However, if updates to multiple tables are likely or required or if the specific update scenarios are unknown, make sure the SEQUENCE clause is included.
INSERT (INSERT with the APPEND hint for loads).
This is a lot more efficient than conventional insert. During loading, disable all constraints and re-enable when finished loading. Note that materialized view logs are required regardless of whether you use direct load or conventional DML.
Try to optimize the sequence of conventional mixed DML operations, direct-path INSERT and the fast refresh of materialized views. You can use fast refresh with a mixture of conventional DML and direct loads. Fast refresh can perform significant optimizations if it finds that only direct loads have occurred, as shown below:
INSERT (SQL*Loader or INSERT /*+ APPEND */) into the detail table
You can use fast refresh with conventional mixed DML (INSERT, UPDATE, and DELETE) to the detail tables. However, fast refresh will be able to perform significant optimizations in its processing if it detects that only inserts or deletes have been done to the tables, such as:
INSERT or DELETE to the detail table
Even more optimal is the separation of INSERT and DELETE.
If possible, refresh should be performed after each type of data change (as shown above) rather than issuing only one refresh at the end. If that is not possible, restrict the conventional DML to the table to inserts only, to get much better refresh performance. Avoid mixing deletes and direct loads.
Furthermore, for refresh ON COMMIT, Oracle keeps track of the type of DML done in the committed transaction. Therefore, do not perform direct-path INSERT and DML to other tables in the same transaction, as Oracle may not be able to optimize the refresh phase.
For ON COMMIT materialized views, where refreshes automatically occur at the end of each transaction, it may not be possible to isolate the DML statements, in which case keeping the transactions short will help. However, if you plan to make numerous modifications to the detail table, it may be better to perform them in one transaction, so that refresh of the materialized view will be performed just once at commit time rather than after each update.
For large loads or refresh, enabling parallel DML will help shorten the length of time for the operation.
You can refresh your materialized views fast after partition maintenance operations on the detail tables. "Partition Change Tracking" for details on enabling PCT for materialized views.
Partitioning the materialized view will also help refresh performance as refresh can update the materialized view using parallel DML. For example, assume that the detail tables and materialized view are partitioned and have a parallel clause. The following sequence would enable Oracle to parallelize the refresh of the materialized view.
DBMS_MVIEW.REFRESH, set the parameter atomic to FALSE. This will use TRUNCATE to delete existing rows in the materialized view, which is faster than a delete.
DBMS_MVIEW.REFRESH with JOB_QUEUES, remember to set atomic to FALSE. Otherwise, JOB_QUEUES will not get used. Set the number of job queue processes greater than the number of processors.
If job queues are enabled and there are many materialized views to refresh, it is faster to refresh all of them in a single command than to call them individually.
REFRESH FORCE to ensure getting a refreshed materialized view that can definitely be used for query rewrite. If a fast refresh cannot be done, a complete refresh will be performed.
If a materialized view contains joins but no aggregates, then having an index on each of the join column rowids in the detail table will enhance refresh performance greatly, because this type of materialized view tends to be much larger than materialized views containing aggregates. For example, consider the following materialized view:
CREATE MATERIALIZED VIEW detail_fact_mv BUILD IMMEDIATE AS SELECT s.rowid "sales_rid", t.rowid "times_rid", c.rowid "cust_rid", c.cust_state_province, t.week_ending_day, s.amount_sold FROM sales s, times t, customers c WHERE s.time_id = t.time_id AND s.cust_id = c.cust_id;
Indexes should be created on columns sales_rid, times_rid and cust_rid. Partitioning is highly recommended, as is enabling parallel DML in the session before invoking refresh, because it will greatly enhance refresh performance.
This type of materialized view can also be fast refreshed if DML is performed on the detail table. It is recommended that the same procedure be applied to this type of materialized view as for a single table aggregate. That is, perform one type of change (direct-path INSERT or DML) and then refresh the materialized view. This is because Oracle can perform significant optimizations if it detects that only one type of change has been done.
Also, Oracle recommends that the refresh be invoked after each table is loaded, rather than load all the tables and then perform the refresh.
For refresh ON COMMIT, Oracle keeps track of the type of DML done in the committed transaction. Oracle therefore recommends that you do not perform direct-path and conventional DML to other tables in the same transaction because Oracle may not be able to optimize the refresh phase. For example, the following is not recommended:
Also, try not to mix different types of conventional DML statements if possible. This would again prevent using various optimizations during fast refresh. For example, try to avoid the following:
If many updates are needed, try to group them all into one transaction because refresh will be performed just once at commit time, rather than after each update.
When you use the DBMS_MVIEW package to refresh a number of materialized views containing only joins with the ATOMIC parameter set to TRUE, if you disable parallel DML, refresh performance may degrade.
In a data warehousing environment, assuming that the materialized view has a parallel clause, the following sequence of steps is recommended:
fact table
ALTER SESSION ENABLE PARALLEL DML statement
Refreshing materialized views containing joins only and single-table aggregate materialized views uses the same algorithms irrespective of whether or not the views are nested. All underlying objects are treated as ordinary tables. If the ON COMMIT refresh option is specified, then all the materialized views are refreshed in the appropriate order at commit time.
Consider the schema in Figure 8-3. Assume all the materialized views are defined for ON COMMIT refresh. If table fact changes, then at commit time you could refresh join_fact_store_time first, and then sum_sales_store_time and join_fact_store_time_prod. No specific order for sum_sales_store_time and join_fact_store_time_prod, because they do not have any dependencies between them.
In other words, Oracle builds a partially ordered set of materialized views and refreshes them such that, after the successful completion of the refresh, all the materialized views are fresh. The status of the materialized views can be checked by querying the appropriate USER_, DBA_, or ALL_MVIEWS view.
If any of the materialized views are defined as ON DEMAND refresh (irrespective of whether the refresh method is FAST, FORCE, or COMPLETE), you will need to refresh them in the correct order (taking into account the dependencies between the materialized views) because the nested materialized view will be refreshed with respect to the current state of the other materialized views (whether fresh or not).
If a refresh fails during commit time, the list of materialized views that has not been refreshed is written to the alert log, and you must manually refresh them along with all their dependent materialized views.
Use the same DBMS_MVIEW procedures on nested materialized views that you use on regular materialized views.
These procedures have the following behavior when used with nested materialized views:
REFRESH is applied to a materialized view my_mv that is built on other materialized views, then my_mv will be refreshed with respect to the current state of the other materialized views (that is, they will not be made fresh first).
REFRESH_DEPENDENT is applied to materialized view my_mv, then only materialized views that directly depend on my_mv will be refreshed (that is, a materialized view that depends on a materialized view that depends on my_mv will not be refreshed).
REFRESH_ALL_MVIEWS is used, the order in which the materialized views will be refreshed is not guaranteed.
GET_MV_DEPENDENCIES provides a list of the immediate (or direct) materialized view dependencies for an object.
After you have performed a load or incremental load and rebuilt the detail table indexes, you need to re-enable integrity constraints (if any) and refresh the materialized views and materialized view indexes that are derived from that detail data. In a data warehouse environment, referential integrity constraints are normally enabled with the NOVALIDATE or RELY options. An important decision to make before performing a refresh operation is whether the refresh needs to be recoverable. Because materialized view data is redundant and can always be reconstructed from the detail tables, it might be preferable to disable logging on the materialized view. To disable logging and run incremental refresh non-recoverably, use the ALTER MATERIALIZED VIEW ... NOLOGGING statement prior to refreshing.
If the materialized view is being refreshed using the ON COMMIT method, then, following refresh operations, consult the alert log alert_<SID>.log and the trace file ora_<SID>_number.trc to check that no errors have occurred.
A major maintenance component of a data warehouse is synchronizing (refreshing) the materialized views when the detail data changes. Partitioning the underlying detail tables can reduce the amount of time taken to perform the refresh task. This is possible because partitioning enables refresh to use parallel DML to update the materialized view. Also, it enables the use of Partition Change Tracking (PCT).
In a data warehouse, changes to the detail tables can often entail partition maintenance operations, such as DROP, EXCHANGE, MERGE, and ADD PARTITION. To maintain the materialized view after such operations in Oracle8i required the use of manual maintenance (see also CONSIDER FRESH) or complete refresh. Oracle9i introduces an addition to fast refresh known as Partition Change Tracking (PCT) refresh.
For PCT to be available, the detail tables must be partitioned. The partitioning of the materialized view itself has no bearing on this feature. If PCT refresh is possible, it will occur automatically and no user intervention is required in order for it to occur.
The following examples will illustrate the use of this feature. In "PCT Fast Refresh Scenario 1", assume sales is a partitioned table using the time_id column and products is partitioned by the prod_category column. The table times is not a partitioned table.
sales table is provided below.
CREATE materialized view LOG on SALES WITH ROWID, SEQUENCE (prod_id, time_id, quantity_sold, amount_sold) INCLUDING NEW VALUES;
CREATE MATERIALIZED VIEW cust_mth_sales_mv BUILD IMMEDIATE REFRESH FAST ON DEMAND ENABLE QUERY REWRITE AS SELECT s.time_id, s.prod_id, SUM(s.quantity_sold), SUM(s.amount_sold), p.prod_name, t.calendar_month_name, COUNT(*), COUNT(s.quantity_sold), COUNT(s.amount_sold) FROM sales s, products p, times t WHERE s.time_id = t.time_id AND s.prod_id = p.prod_id GROUP BY t.calendar_month_name, s.prod_id, p.prod_name, s.time_id;
DBMS_MVIEW.EXPLAIN_MVIEW procedure to determine which tables will allow PCT refresh.
MVNAME CAPABILITY_NAME POSSIBLE RELATED_TEXT MSGTXT ----------------- --------------- -------- ------------ ---------------- CUST_MTH_SALES_MV PCT Y SALES CUST_MTH_SALES_MV PCT_TABLE Y SALES CUST_MTH_SALES_MV PCT_TABLE N PRODUCTS no partition key or PMARKER in SELECT list CUST_MTH_SALES_MV PCT_TABLE N TIMES relation is not a partitioned table
As can be seen from the partial sample output from EXPLAIN_MVIEW, any partition maintenance operation performed on the sales table will allow PCT fast refresh. However, PCT is not possible after partition maintenance operations or updates to the products table as there is insufficient information contained in cust_mth_sales_mv for PCT refresh to be possible. Note that the times table is not partitioned and hence can never allow for PCT refresh. Oracle will apply PCT refresh if it can determine that the materialized view has sufficient information to support PCT for all the updated tables.
SPLIT operation of one partition in the sales table becomes necessary.
ALTER TABLE SALES SPLIT PARTITION month3 AT (TO_DATE('05-02-1998', 'DD-MM-YYYY')) INTO ( PARTITION month3_1 TABLESPACE summ, PARTITION month3 TABLESPACE summ );
sales table.
cust_mth_sales_mv using the DBMS_MVIEW.REFRESH procedure.
EXECUTE DBMS_MVIEW.REFRESH('CUST_MTH_SALES_MV', 'F', '',TRUE,FALSE,0,0,0,FALSE);
Fast refresh will automatically do a PCT refresh as it is the only fast refresh possible in this scenario. However, fast refresh will not occur if a partition maintenance operation occurs when any update has taken place to a table on which PCT is not enabled. This is shown in "PCT Fast Refresh Scenario 2".
"PCT Fast Refresh Scenario 1" would also be appropriate if the materialized view was created using the PMARKER clause as illustrated below.
CREATE MATERIALIZED VIEW cust_sales_marker_mv BUILD IMMEDIATE REFRESH FAST ON DEMAND ENABLE QUERY REWRITE AS SELECT DBMS_MVIEW.PMARKER(s.rowid) s_marker, SUM(s.quantity_sold), SUM(s.amount_sold), p.prod_name, t.calendar_month_name, COUNT(*), COUNT(s.quantity_sold), COUNT(s.amount_sold) FROM sales s, products p, times t WHERE s.time_id = t.time_id AND s.prod_id = p.prod_id GROUP BY DBMS_MVIEW.PMARKER(s.rowid), p.prod_name, t.calendar_month_name;
In "PCT Fast Refresh Scenario 2", the first three steps are the same as in "PCT Fast Refresh Scenario 1". Then, the SPLIT partition operation to the sales table is performed, but before the materialized view refresh occurs, records are inserted into the times table.
SPLIT operation, as shown in "PCT Fast Refresh Scenario 1", some data will be inserted into the times table.
ALTER TABLE SALES SPLIT PARTITION month3 AT (TO_DATE('05-02-1998', 'DD-MM-YYYY')) INTO ( PARTIITION month3_1 TABLESPACE summ, PARTITION month3 TABLESPACE summ);
cust_mth_sales_mv.
EXECUTE DBMS_MVIEW.REFRESH('CUST_MTH_SALES_MV', 'F', '',TRUE,FALSE,0,0,0,FALSE); ORA-12052: cannot fast refresh materialized view SH.CUST_MTH_SALES_MV
The materialized view is not fast refreshable because DML has occurred to a table on which PCT fast refresh is not possible. To avoid this occurring, Oracle recommends performing a fast refresh immediately after any partition maintenance operation on detail tables for which partition tracking fast refresh is available.
If the situation in "PCT Fast Refresh Scenario 2" occurs, there are two possibilities; perform a complete refresh or switch to the CONSIDER FRESH option outlined below, if suitable. However, it should be noted that CONSIDER FRESH and partition change tracking fast refresh are not compatible. Once the ALTER MATERIALIZED VIEW cust_mth_sales_mv CONSIDER FRESH statement has been issued, PCT refresh will not longer be applied to this materialized view, until a complete refresh is done.
A common situation in a warehouse is the use of rolling windows of data. In this case, the detail table and the materialized view may contain say the last 12 months of data. Every month, new data for a month is added to the table and the oldest month is deleted (or maybe archived). PCT refresh provides a very efficient mechanism to maintain the materialized view in this case.
ALTER TABLE sales ADD PARTITION month_new ... ALTER TABLE sales EXCHANGE PARTITION month_new month_new_table
ALTER TABLE sales DROP PARTITION month_oldest;
EXECUTE DBMS_MVIEW.REFRESH('CUST_MTH_SALES_MV', 'F', '', TRUE, FALSE,0,0,0,FALSE);
Fast refresh will automatically detect that PCT is available and perform a PCT refresh.
If the materialized view and a detail table have the same partitioning criteria, then you could use CONSIDER FRESH to maintain the materialized view after partition maintenance operations.
The following example demonstrates how you can manually maintain an unsynchronized detail table and materialized view. Assume the sales table and the cust_mth_sales_mv are partitioned identically, and contain say 12 months of data, one month in each partition.
ALTER TABLE sales DROP PARTITION month_oldest;
ALTER MATERIALIZED VIEW cust_mth_sales_mv DROP PARTITION month_oldest;
CONSIDER FRESH to declare that the materialized view has been refreshed.
ALTER MATERIALIZED VIEW cust_mth_sales_mv CONSIDER FRESH;
In a data warehouse, you may often wish to accumulate historical information in the materialized view even though this information is no longer in the detailed tables. In this case, you could maintain the materialized view using the ALTER MATERIALIZED VIEW <materialized view name> CONSIDER FRESH statement.
Note that CONSIDER FRESH declares that the contents of the materialized view are FRESH (in sync with the detail tables). Care must be taken when using this option in this scenario in conjunction with query rewrite because you may see unexpected results.
After using CONSIDER FRESH in an historical scenario, you will be able to apply traditional fast refresh after DML and direct loads to the materialized view, but not PCT fast refresh. This is because if the detail table partition at one time contained data that is currently kept in aggregated form in the materialized view, PCT refresh in attempting to resynchronize the materialized view with that partition could delete historical data which cannot be recomputed.
Assume the sales table stores the prior year's data and the cust_mth_sales_mv keeps the prior 10 years of data in aggregated form.
sales table:
ALTER TABLE sales TRUNCATE PARTITION month1;
The materialized view is now considered stale and requires a refresh because of the partition operation. However, as the detail table no longer contains all the data associated with the partition fast refresh cannot be attempted.
ALTER MATERIALIZED VIEW cust_mth_sales_mv CONSIDER FRESH;
This statement informs Oracle that cust_mth_sales_mv is fresh for your purposes. However, the materialized view now has a status that is neither known fresh nor known stale. Instead, it is UNKNOWN. If the materialized view has query rewrite enabled in QUERY_REWRITE_INTEGRITY=STALE_TOLERATED mode it will be used for rewrite.
sales.
EXECUTE DBMS_MVIEW.REFRESH('CUST_MTH_SALES_MV', 'F', '', TRUE, FALSE,0,0,0,FALSE);
Because the fast refresh detects that only INSERT statements occurred against the sales table it will update the materialized view with the new data. However, the status of the materialized view will remain UNKNOWN. The only way to return the materialized view to FRESH status is with a complete refresh which, also will remove the historical data from the materialized view.
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