Which column gains the least from a B-tree index of its own?

Options

• ✗ email, looked up to log users in
• ✗ created_at, used for "last 7 days" range queries
• ✗ user_id on an orders table, used to list one user's orders
• ✓ is_active, true for about half the rows Correct

Why

An index pays off when it narrows a query to a small fraction of the rows. A filter matching half the table reads so many rows that a full scan is cheaper, and the planner will usually ignore the index while every write still pays to maintain it.

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