Storing and Purging Cache for SAPBW Data Sources

Managing Performance Tuning and Query Caching 7-21

7.7 Strategies for Using the Cache

One of the main advantages of query caching is to improve apparent query performance. It might be valuable to seed the cache during off hours by running queries and caching their results. A good seeding strategy requires that you know when cache hits occur. To seed the cache for all users, then you might seed the cache with the following query: SELECT User, SRs After seeding the cache using SELECT User, SRs, the following queries are cache hits: SELECT User, SRs WHERE user = valueofnq_SESSION.USER and the user was USER1 SELECT User, SRs WHERE user = valueofnq_SESSION.USER and the user was USER2 SELECT User, SRs WHERE user = valueofnq_SESSION.USER and the user was USER3 This section contains the following topics: ■ Section 7.7.1, About Cache Hits ■ Section 7.7.2, Running a Suite of Queries to Populate the Cache ■ Section 7.7.3, Using Agents to Seed the Oracle BI Server Cache ■ Section 7.7.4, Using the Cache Manager

7.7.1 About Cache Hits

When caching is enabled, each query is evaluated to determine whether it qualifies for a cache hit. A cache hit means that the server was able to use cache to answer the query and did not go to the database at all. The Oracle BI Server can use the query cache to answer queries at the same or higher level of aggregation. Many factors determine whether cache is hit. Table 7–3 describes these factors. 7-22 System Administrators Guide for Oracle Business Intelligence Enterprise Edition Table 7–3 Factors That Determine Whether Cache Is Hit Factor or Rule Description A subset of columns in the SELECT list must match All of the columns in the SELECT list of a new query have to exist in the cached query to qualify for a cache hit, or they must be able to be calculated from the columns in the query. This rule describes the minimum requirement to hit the cache, but meeting this rule does not guarantee a cache hit. The other rules listed in this table also apply. Columns in the SELECT list can be composed of expressions on the columns of the cached queries The Oracle BI Server can calculate expressions on cached results to answer the new query, but all the columns must be in the cached result. For example, the query: SELECT product, month, averageprice FROM sales WHERE year = 2000 hits cache on the query: SELECT product, month, dollars, unitsales FROM sales WHERE year = 2000 because averageprice can be computed from dollars and unitsales averageprice = dollarsunitsales. WHERE clause must be semantically the same or a logical subset For the query to qualify as a cache hit, the WHERE clause constraints must be either equivalent to the cached results, or a subset of the cached results. A WHERE clause that is a logical subset of a cached query qualifies for a cache hit if the subset meets one of the following criterion: ■ A subset of IN list values. Queries requesting fewer elements of an IN list cached query qualify for a cache hit. For example, the following query: SELECT employeename, region FROM employee, geography WHERE region in EAST, WEST qualifies as a hit on the following cached query: SELECT employeename, region FROM employee, geography WHERE region in NORTH, SOUTH, EAST, WEST ■ It contains fewer but identical OR constraints than the cached result. ■ It contains a logical subset of a literal comparison. For example, the following predicate: WHERE revenue 1000 qualifies as a cache hit on a comparable query with the predicate: WHERE revenue 5000 ■ There is no WHERE clause. If a query with no WHERE clause is cached, then queries that satisfy all other cache hit rules qualify as cache hits regardless of their WHERE clause. Dimension-only queries must be an exact match If a query is dimension only, meaning that no fact or measure is included in the query, then only an exact match of the projection columns of the cached query hits the cache. This behavior prevents false positives when there are multiple logical sources for a dimension table. Queries with special functions must be an exact match Other queries that contain special functions such as time series functions AGO, TODATE, and PERIODROLLING, external aggregation functions, and filter metrics must also be an exact match with the projection columns in the cached query. In these cases, the filter must also be an exact match. Set of logical tables must match To qualify as a cache hit, all incoming queries must have the same set of logical tables as the cache entry. This rule avoids false cache hits. For example, SELECT FROM product does not match SELECT FROM product, sales. Managing Performance Tuning and Query Caching 7-23

7.7.1.1 Ensuring Correct Cache Results When Using Row-Level Database Security

When using a row-level database security strategy, such as a Virtual Private Database VPD, the returned data results are contingent on the authorization credentials of the user. Because of this, the Oracle BI Server must know whether a data source is using row-level database security and which variables are relevant to security. To ensure that cache hits only occur on cache entries that include and match all security-sensitive variables, you must correctly configure the database object and session variable objects in the Administration Tool, as follows: ■ Database object. In the Physical layer, in the General tab of the Database dialog, select Virtual Private Database to specify that the data source is using row-level database security. If you are using row-level database security with shared caching, then you must select this option to prevent the sharing of cache entries whose security-sensitive variables do not match. Session variable values must match, including security session variables If the logical SQL or physical SQL statement refers to any session variable, then the session variable values must match. Otherwise, the cache is not hit. In addition, the value of session variables that are security sensitive must match the security session variable values that are defined in the repository, even though the logical SQL statement itself does not reference session variables. See Section 7.7.1.1, Ensuring Correct Cache Results When Using Row-Level Database Security for more information. Equivalent join conditions The resultant joined logical table of a new query request has to be the same as or a subset of the cached results to qualify for a cache hit. DISTINCT attribute must be the same If a cached query eliminates duplicate records with DISTINCT processing for example, SELECT DISTINCT..., then requests for the cached columns must also include the DISTINCT processing; a request for the same column without the DISTINCT processing is a cache miss. Queries must contain compatible aggregation levels Queries that request an aggregated level of information can use cached results at a lower level of aggregation. For example, the following query requests the quantity sold at the supplier and region and city level: SELECT supplier, region, city, qtysold FROM suppliercity The following query requests the quantity sold at the city level: SELECT city, qtysold FROM suppliercity The second query results in a cache hit on the first query. Limited additional aggregation For example, if a query with the column qtysold is cached, then a request for RANKqtysold results in a cache miss. Additionally, a query that requests qtysold at the country level can get a cache hit from a query that requests qtysold at the country, region level. ORDER BY clause must be comprised of columns in the select list Queries that order by columns that are not contained in the select list result in cache misses. Avoiding cache misses using advanced hit detection You can avoid some cache misses by setting the parameter USE_ADVANCED_HIT_ DETECTION to YES in the NQSConfig.INI file. Advanced hit detection enables an expanded search of the cache for hits. See USE_ADVANCED_HIT_DETECTION for more information. Table 7–3 Cont. Factors That Determine Whether Cache Is Hit Factor or Rule Description