Apex Developer Guide

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Apex Developer Guide
Release Notes
Getting Started with Apex
Writing Apex
Data Types and Variables
Control Flow Statements
Classes, Objects, and Interfaces
Working with Data in Apex
Working with sObjects
Data Manipulation Language
How DML Works
Adding and Retrieving Data With DML
DML Statements vs. Database Class Methods
DML Operations As Atomic Transactions
DML Operations
Exception Handling
More About DML
Locking Records
Locking Statements
Locking in a SOQL For Loop
Avoiding Deadlocks
SOQL and SOSL Queries
SOQL For Loops
sObject Collections
Dynamic Apex
Apex Security and Sharing
Custom Settings
Running Apex
Debugging, Testing, and Deploying Apex
Apex Reference
Appendices
PDF

Avoiding Deadlocks

Apex has the possibility of deadlocks, as does any other procedural logic language involving updates to multiple database tables or rows. To avoid such deadlocks, the Apex runtime engine:
  1. First locks sObject parent records, then children.
  2. Locks sObject records in order of ID when multiple records of the same type are being edited.

As a developer, use care when locking rows to ensure that you are not introducing deadlocks. Verify that you are using standard deadlock avoidance techniques by accessing tables and rows in the same order from all locations in an application.