A Salesforce org grew from 50,000 Cases to 5 million Cases over four years. Apex code that processed Cases used to complete in 200ms. The same code now takes 30 seconds when it completes at all; sometimes it times out. The team blames "the platform getting slower." The actual cause is almost always SOQL selectivity. The query no longer hits an index. The platform is scanning the full table.
Selectivity is the single most important performance concern in Apex code touching large objects. The fix is mechanical when caught early and expensive when caught after the data has accumulated. Sapota's Salesforce team treats LDV (large data volume) audits as a recurring engagement type because the failure mode keeps recurring.
What selectivity actually means
A SOQL query is selective when Salesforce can use an index to find the matching records quickly. A query is non-selective when Salesforce has to scan the full table.
Salesforce indexes:
