• Pradip
• 28 Nov, 2025
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• 3 Mins Read

Understanding Latches, Locks & Enqueues in Oracle

Understanding Latches, Locks & Enqueues in Oracle (Complete Guide for DBAs)

When multiple users access data simultaneously, Oracle uses different concurrency control mechanisms to maintain data consistency, transaction isolation, and instance stability.
Among these mechanisms, latches, locks, and enqueues are the most important — yet often the most confusing for new DBAs.

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1. What Are Latches in Oracle?

Latches are low-level serialization mechanisms used to protect in-memory shared data structures in the SGA.

Think of a latch as a very fast, short-duration lock used internally by Oracle.

Key Characteristics

• Lightweight and in-memory

• Very fast (microseconds)

• Not transactional

• Held for a very short time

• Protect shared memory structures (e.g., library cache, buffer cache)

Real-World Example

When multiple sessions try to read or write from the buffer cache, Oracle uses buffer cache latches to avoid corruption.

Common Latch Contention Symptoms

• High CPU usage

• Library Cache: Pin or Lock latch waits

• Query execution delays

Useful Views

Latches = Speed + Protection (not related to user transactions)

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2. What Are Locks in Oracle?

Locks are used to control access to data in the database and maintain transaction isolation.

These are DML locks that ensure two users don’t change the same data at the same time.

Key Characteristics

• Protect rows, tables, and objects

• Held for the duration of a transaction

• Enforce data consistency

• Acquired automatically by Oracle

Example

Session A updates a row:

Session B tries to update the same row → row lock wait.

Types of Locks

• Row-level locks (TX)

• Table locks (TM)

• DDL locks

• Internal locks

Useful Views

Locks = Protecting data during transactions

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3. What Are Enqueues in Oracle?

Enqueues are more advanced, flexible locking mechanisms that follow a queue structure — hence the name “enqueue”.

They are used for transactional locks, schema locks, and inter-instance coordination (RAC).

Key Characteristics

• Follow a queue mechanism

• Can be held in different modes (NULL, Share, Exclusive, etc.)

• Can persist for longer periods

• Protects multiple database resources

Example

• When a session updates a row → TX enqueue

• When a session executes DDL → TM enqueue

Common Enqueue Types

Useful View

Enqueues = Structured, queued locks for multiple resource types

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4. Latches vs Locks vs Enqueues (Quick Comparison)

Simple Explanation:

• Latches → Protect RAM

• Locks → Protect Data

• Enqueues → Protect Resources

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5. Common Wait Events Related to Each

Latch Waits

• latch: library cache

• latch: shared pool

Lock Waits

• enq: TX - row lock contention

• enq: TM - contention

Enqueue Waits

• enq: TM – contention

• enq: HW – contention

• enq: CF – contention

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6. How to Diagnose Contention

 Check Blocking and Waiting Sessions

 Find the Object Causing Lock

 Kill a Blocking Session

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Conclusion

Understanding the difference between latches, locks, and enqueues is essential for any Oracle DBA.
They help keep the database consistent, stable, and performant, even when thousands of sessions are running concurrently.

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