Understanding how the Salesforce Query Optimiser (SQO) works is fundamental to delivering performance at the data layer. Good practice in this regard, as part of an optimised physical data model implementation, is key to delivering efficient data access at the application layer.
In short, certain fields are indexed by default, custom indexes can be added via External Id and unique field attributes or by Salesforce support. For composite indexes the latter is required. The SQO utilises an index statistics table which records the distribution of data in each index. When processing a query, a pre-query is made to the statistics table to determine whether a candidate index is useful or not, in terms of selectivity thresholds etc.
The Query & Search Optimization Cheat Sheet provides a very useful reference in terms of the logic applied by the SQO, plus the fields indexed by default. Bookmark as a guide next time you’re writing a complex SOQL query or designing a data model. In the latter case performance of data retrieval is an often overlooked design consideration.