Database Animations: The Interview Question Everybody Gets Wrong

A common SQL Server interview question about which column should go first in a composite index is challenged: the popular answer of ordering by which column has more distinct (selective) values is wrong. Using the Stack Overflow database's Users table as an example, equality searches on two columns perform identically regardless of column order because SQL Server can seek on both. Column order only matters when inequality searches are involved - leading with the equality-filtered column dramatically narrows the search space, while leading with the range-filtered column effectively forces a scan disguised as a seek. The real principle is that filter order should be driven by the query's filtering operators and comparison values, not by column selectivity or a blanket 'equality before inequality' rule.

Source: https://www.brentozar.com/archive/2026/09/database-animations-the-interview-question-everybody-gets-wrong. 8 Sync News only summarizes and links out; content copyright belongs to the authors and original sources.