Oracle SQL Optimization - Intro
Identifying Poorly Performing SQL in Oracle: A Developer's Guide
Author: Database Engineering Team
This technical guide explains practical, developer-focused techniques for identifying and diagnosing poorly performing SQL statements in Oracle databases. It covers built-in Oracle performance views and tools, shows example queries, and provides guidance on interpreting execution plans, wait events, and prioritization criteria to focus tuning efforts where they matter most. +1
Overview
Poorly performing SQL is one of the most common root causes of slow applications in Oracle environments. Identifying these statements early allows developers to reduce response times, lower database load, and improve overall system stability. +1
Oracle provides several built-in performance monitoring views and reports that make it possible to find high-impact SQL without third-party tools. This document focuses on practical usage of AWR, V$SQL, ASH, and SQL Monitor from a developer perspective. +1
Why SQL Performance Matters
Inefficient SQL can consume excessive CPU, memory, and I/O, directly impacting other sessions on the same database. A single bad query can degrade the performance of an entire application. From a development standpoint, understanding SQL performance helps: +1
Prevent scalability issues.
Reduce operational costs.
Shorten incident resolution time when performance problems occur in production.
Top Methods to Identify Poorly Performing SQL
Oracle offers multiple complementary tools to analyze SQL performance. Each tool answers different questions, such as which SQL uses the most resources over time or which SQL is slow right now.
| Tool | Primary Use Case | Typical Scenario |
|---|---|---|
| AWR (Automatic Workload Repository) | Historical analysis of top SQL by resource usage | Investigating performance issues that occurred in the past |
| V$SQL | Current and recent SQL execution statistics | Finding SQL with high CPU or elapsed time right now |
| ASH (Active Session History) | Session-level wait and activity analysis | Understanding where time is spent during slow periods |
| SQL Monitor | Detailed execution monitoring for individual SQL | Diagnosing long-running or parallel queries |
Using AWR to Find High-Impact SQL
AWR reports rank SQL statements by metrics such as elapsed time, CPU time, logical reads, and physical reads. These rankings help identify SQL that contributes most to overall database load. Developers commonly start with the 'SQL ordered by Elapsed Time' or 'SQL ordered by CPU Time' sections of the AWR report to locate tuning candidates. +2
=== Using VSQLforReal−TimeAnalysis===[cite s
tart]TheVSQL view provides execution statistics for SQL statements currently in the shared pool. It is useful for identifying SQL with high average execution time or excessive buffer gets. Developers should focus on per-execution metrics rather than total values to avoid being misled by frequently executed but inexpensive SQL.
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| Metric | Description |
|---|---|
| ELAPSED_TIME / EXECUTIONS | Average elapsed time per execution in microseconds |
| BUFFER GETS / EXECUTIONS | Logical I/O per execution |
| DISK_READS / EXECUTIONS | Physical reads per execution |
Using ASH to Understand Waits
Active Session History (ASH) samples active database sessions and records what they are waiting for. ASH is especially useful when performance problems are intermittent. By grouping ASH data by SQL_ID and wait event, developers can identify which SQL statements are blocked by I/O, locks, or CPU contention. +2
Using SQL Monitor for Long-Running SQL
SQL Monitor automatically captures execution details for SQL statements that run longer than a threshold or use parallel execution. It provides step-by-step execution progress, row counts, and time spent in each operation, making it ideal for diagnosing complex queries. +1
Sample Queries
The following examples illustrate how developers can query Oracle dynamic performance views to identify poorly performing SQL.
| Purpose | Example Query |
|---|---|
| Find SQL with highest average elapsed time | SELECT sql_id, executions,
elapsed_time/executions avg_elapsed
FROM vsqlWHEREexecutions>0ORDERBYavg
e
lapsedDESCs tart]FindSQLwithhighestbuffergetsperexecution[cite:39]∣<syntaxhighlightlang="sql">SELECTsql i d,buffer g ets/executionsavg g etsFROMvsql WHERE executions > 0 ORDER BY avg_gets DESC </syntaxhighlight> |
Execution Plan Analysis
Execution plans explain how Oracle accesses data and joins tables. Poor performance often correlates with inefficient access paths or join methods. Developers should look for: +1
Full table scans on large tables.
Unexpected nested loop joins.
Significant differences between estimated and actual row counts.
Common Red Flags and Wait Events
Certain patterns frequently indicate SQL that requires tuning. These red flags can often be spotted directly in execution plans or performance views.
| Red Flag | Why It Matters |
|---|---|
| Full table scan on large tables | Causes excessive I/O and CPU usage |
| High buffer gets per execution | Indicates inefficient data access |
| Large difference between estimated and actual rows | Leads to suboptimal execution plans |
| Frequent hard parsing | Increases CPU usage and latch contention |
Wait events provide insight into why SQL is slow.
| Wait Event | Typical Cause |
|---|---|
| db file sequential read | Single-block I/O, often index access |
| db file scattered read | Multi-block I/O, often full table scans |
| CPU + Wait for CPU | CPU contention due to inefficient SQL |
| enq: TX - row lock contention | Blocking due to concurrent DML |
Prioritization Checklist
Not all slow SQL should be tuned first. Developers should prioritize based on business impact and resource consumption.
| Criterion | Question to Ask |
|---|---|
| Execution frequency | Is this SQL executed thousands of times per hour? |
| User impact | Does it affect critical user-facing functionality? |
| Resource usage | Does it consume significant CPU or I/O? |
| Ease of fix | Can indexing or query rewrite provide quick gains? |
