Fixing Common Issues with Adaptive Cursor Sharing in Oracle Database

Adaptive Cursor Sharing (ACS) is a powerful feature in Oracle Database that enhances performance by dynamically adjusting the way SQL statements are planned based on the runtime conditions. This feature is particularly useful in environments with varying data distributions and workloads. However, like any sophisticated system, it can encounter issues that may hinder performance or lead to undesirable execution plans. In this blog post, we will delve deep into how Adaptive Cursor Sharing functions and provide actionable solutions to common errors you may face.

Understanding Adaptive Cursor Sharing

Before diving into troubleshooting, it's crucial to understand the basics of Adaptive Cursor Sharing. When a SQL statement is executed, the Oracle optimizer generates a specific execution plan. With ACS, this plan can adapt based on different bind variable values during subsequent executions. The optimizer evaluates the incoming data distributions and selects the most efficient execution plan.

How It Works

When a SQL statement is first executed, the optimizer creates a cursor for that statement. With ACS, if it detects changing execution conditions, it can create additional child cursors associated with the parent. For example, consider the following SQL statement:

SELECT *
FROM employees
WHERE department_id = :dept_id
AND salary > :min_salary;

When executed with different :dept_id and :min_salary values, the optimizer can create multiple execution plans, optimizing for each set of inputs. This ensures that queries run efficiently and reduces resource consumption, paving the way for smoother database operations.

Types of Cursor Sharing

1. Exact: Uses the exact same execution plan for identical SQL statements regardless of varying bind values.
2. Similar: Allows SQL statements to share execution plans but only if they are "similar" (e.g., similar bind variable values).
3. Adaptive: This is the default behavior for ACS, enabling the optimizer to switch plans based on execution context dynamically.

Common Issues With Adaptive Cursor Sharing

While ACS can greatly improve performance, several issues can arise. Below are some common problems and how to fix them.

1. High Resource Consumption

Problem

An increase in the number of child cursors can lead to high memory and processing demands.

Solution

Monitor and adjust the parameters that govern cursor creation. Here’s how you can track cursor usage:

SELECT sql_id, COUNT(*)
FROM v$sql
GROUP BY sql_id
HAVING COUNT(*) > 1
ORDER BY COUNT(*) DESC;

This query will help identify SQL statements generating multiple child cursors. You can then assess the need for tuning or consolidating SQL execution strategies.

2. Inefficient Execution Plans

Problem

The optimizer may choose a non-optimal execution plan for certain bind values.

Solution

You can use hints to guide the optimizer in generating a more appropriate plan. For instance:

SELECT /*+ USE_NL(e) */ *
FROM employees e
WHERE e.department_id = :dept_id
AND e.salary > :min_salary;

The USE_NL hint instructs the optimizer to prefer a nested loop for joins, which can be more efficient under specific conditions.

Additionally, consider employing the CURSOR_SHARING parameter. Setting this at the session level could help:

ALTER SESSION SET CURSOR_SHARING=FORCE;

Index usage can significantly impact plans, so perform an index analysis regularly. Create or modify indexes based on application usage patterns.

3. Bind Variable Peeking Issues

Problem

The optimizer may misjudge the optimal execution plan due to bind variable peeking—where it optimizes based on initial execution variables and fails to adjust later.

Solution

You can mitigate this issue by using bind variable peeking based on system changes. Check if you have the correct configuration:

SHOW PARAMETER bind_variable_peeking;

If it's set to off, enabling it can enhance performance:

ALTER SYSTEM SET bind_variable_peeking=TRUE SCOPE=BOTH;

4. Invalid SQL Execution Paths

Problem

If old statistics are utilized, the optimizer may choose a sub-par execution path.

Solution

Perform regular updates on database statistics using:

EXEC DBMS_STATS.GATHER_SCHEMA_STATS('HR');

This ensures that the optimizer has the most current distribution of data, which is essential for optimal performance.

5. Incorrectly Configured Cursor Sharing

Problem

If cursor sharing is not correctly configured, it can lead to performance problems.

Solution

Evaluate the configuration settings and adjust accordingly. For a mixed workload, consider setting the cursor sharing to FORCE or ADAPTIVE. Here's how to check and change the parameter:

SHOW PARAMETER CURSOR_SHARING;
ALTER SYSTEM SET CURSOR_SHARING=FORCE SCOPE=BOTH;

Ensure this action aligns with your application requirements, as not every application benefits from forced cursor sharing.

Monitoring and Optimization

Using AWR Reports

A valuable tool for diagnosing issues with ACS and overall performance is the AWR (Automatic Workload Repository) report. Use the following SQL to generate a report:

SELECT * FROM DBA_HIST_SNAPSHOT;

This will provide insights into your SQL usage over time. Focus on SQL that frequently rebinds variables, identifying statements that may benefit from either better indexing or SQL refinement.

Setting Up Alerts

You can set up alerts in Oracle to notify you when cursor counts exceed defined thresholds. This proactive approach allows for quick remediation of growing performance issues before they become detrimental.

Deep Dive: Code Optimization Example

Here’s a more complex example regarding execution plans and ACS:

CREATE TABLE projects (
    project_id NUMBER,
    project_name VARCHAR2(100),
    department_id NUMBER
);

INSERT INTO projects (project_id, project_name, department_id)
VALUES (1, 'Project A', 10);

-- Using statistics
EXEC DBMS_STATS.GATHER_TABLE_STATS('YOUR_SCHEMA', 'PROJECTS');

SELECT *
FROM projects
WHERE department_id = :dept_id;

Start by gathering statistics after the initial data loads. Adjust your queries based on the expected workload, checking for performance bottlenecks regularly.

Closing Remarks

Adaptive Cursor Sharing is a remarkable feature of the Oracle Database that allows for efficient execution plans in a dynamic environment. However, as with any system, issues can arise, affecting performance and resource utilization. By familiarizing yourself with common issues and their solutions, you can maintain an efficient, high-performing database system.

For further reading on Oracle Database performance tuning and ACS, check out:

• Oracle Adaptive Execution Plans
• Oracle Database Performance Tuning Guide

By proactively monitoring, adjusting configurations, and tuning SQL statements, you can ensure that Adaptive Cursor Sharing serves its purpose effectively. Happy tuning!