Apex Developer Guide

Getting Started
Writing Apex
Data Types and Variables
Control Flow Statements
Classes, Objects, and Interfaces
Working with Data in Apex
sObject Types
Adding and Retrieving Data
DML
SOQL and SOSL Queries
Working with SOQL and SOSL Query Results
Accessing sObject Fields Through Relationships
Understanding Foreign Key and Parent-Child Relationship SOQL Queries
Working with SOQL Aggregate Functions
Working with Very Large SOQL Queries
Using SOQL Queries That Return One Record
Improving Performance by Not Searching on Null Values
Working with Polymorphic Relationships in SOQL Queries
Using Apex Variables in SOQL and SOSL Queries
Querying All Records with a SOQL Statement
SOQL For Loops
sObject Collections
Dynamic Apex
Apex Security and Sharing
Custom Settings
Ways to Invoke Apex
Finishing Touches
Reference
Appendices
Glossary
PDF

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Improving Performance by Not Searching on Null Values

In your SOQL and SOSL queries, avoid searching records that contain null values. Filter out null values first to improve performance. In the following example, any records where the treadID value is null are filtered out of the returned values.