2 - 3 minutes readWhat is result cache in oracle Database with use case example?

In Oracle Database, the Result Cache is a feature that allows the caching of the results of SQL queries in memory. This caching mechanism can improve query performance for frequently executed queries by storing the query result set in memory, reducing the need to recompute the result for identical queries.

Here’s a use case example to illustrate the concept of the Result Cache:

**Use Case: Employee Salary Calculation**

Consider a scenario where you have an Oracle database containing a table named `Employee` with the following structure:

“`sql
CREATE TABLE Employee (
EmployeeID NUMBER PRIMARY KEY,
FirstName VARCHAR2(50),
LastName VARCHAR2(50),
Salary NUMBER
);
“`

You frequently need to calculate the total salary of all employees. You have a query for this purpose:

“`sql
SELECT SUM(Salary) FROM Employee;
“`

This query calculates the total salary by summing up the `Salary` column for all employees in the `Employee` table.

Now, let’s consider two scenarios:

**Scenario 1: Without Result Cache**

In this scenario, you execute the query to calculate the total salary multiple times throughout the day. Each time the query is executed, Oracle performs the following steps:

1. Parses the SQL query.
2. Optimizes the query and generates an execution plan.
3. Retrieves the data from the `Employee` table.
4. Calculates the sum of salaries.
5. Returns the result.

This process happens every time the query is executed, even though the data in the `Employee` table has not changed between executions. It can lead to unnecessary overhead, especially if the table contains a large number of rows.

**Scenario 2: With Result Cache**

In this scenario, you enable the Result Cache for the query:

“`sql
SELECT /*+ RESULT_CACHE */ SUM(Salary) FROM Employee;
“`

By adding the `/*+ RESULT_CACHE */` hint to the query, you instruct Oracle to cache the result of this query in memory.

Here’s how the process works:

1. The first time you execute the query, Oracle performs the same steps as in Scenario 1 but also caches the result set in memory.
2. Subsequent executions of the same query do not require re-parsing, re-optimizing, or re-retrieving the data. Instead, Oracle checks if the same query has been executed with the same underlying data. If it has, Oracle retrieves the result from the cache in memory, significantly reducing the query execution time.
3. If the underlying data in the `Employee` table changes (e.g., salaries are updated), Oracle automatically invalidates the cached result for this query to ensure data consistency. The next execution of the query will recompute the result and cache it again.

In this use case, the Result Cache improves query performance by eliminating the need to recompute the sum of salaries for every execution of the query, as long as the data remains unchanged. It is particularly beneficial for queries that are frequently executed with the same parameters against static or slowly changing data.