1. Administering Oracle Analytics Server
  2. Scale and Deploy for High Availability and Performance
  3. Manage Performance Tuning and Query Caching
  4. Strategies for Using the Cache
  5. About Cache Hits

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 Oracle BI Server was able to use cache to answer the query and didn't go to the database at all. 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. The table below describes these factors.

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 doesn't 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

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 = dollars/unitsales).

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's 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.

In addition columns that are used on the WHERE clause must be on the projection list. For example, the following query: 

SELECT employeename
FROM employee, geography
WHERE region in ('EAST', 'WEST')

Doesn't result in a cache hit for the seeding query in the previous list because REGION isn't on the projection list.

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), limit and offset functions (OFFSET and FETCH), relationship functions (ISANCESTOR, ISLEAF, ISROOT, and ISSIBLING), external aggregation functions, and generally 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. For filter metrics, if the filter metric can be rewritten as a WHERE clause, then the subset cache might be leveraged.

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 doesn't match SELECT * FROM product, sales.

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 isn't hit.

In addition, the value of session variables that are security sensitive must match the security session variable values that are defined in the semantic model, even though the logical SQL statement itself doesn't reference session variables. See Ensure Correct Cache Results When Using Row-Level Database Security.

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 RANK(qtysold) 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 aren't 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_DETECTIONUSE_ADVANCED_HIT_DETECTION.

Diagnosing cache hit behavior

To better assess cache hit behavior, set the ENABLE_CACHE_DIAGNOSTICS session variable to 4, as shown in the following example:

ENABLE_CACHE_DIAGNOSTICS=4

Ensure 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, 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 Model 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're 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 don't match.

  • Session Variable object. For security-related variables, in the Session Variable dialog, select Security Sensitive to identify them as sensitive to security when using a row-level database security strategy. This option ensures that cache entries are marked with the security-sensitive variables, enabling security-sensitive variable matching on all incoming queries.

Refer to the following resources: