SQLite Indexing Strategies Mastery Hub: The Industry Practic

In SQLite, when considering an index on a composite key (e.g., `CREATE INDEX idx_a_b ON table_name (col_a, col_b);`), which of the following query patterns would benefit most directly and significantly from this index?

A developer is experiencing slow query performance on a table with millions of rows, specifically with queries involving range scans on a single column. They have created a B-tree index on this column. However, the query planner is still choosing a full table scan. What is a likely reason for this behavior, assuming the index is correctly defined?

Consider a scenario where a table `orders` has columns `customer_id`, `order_date`, and `order_total`. A common query is `SELECT SUM(order_total) FROM orders WHERE customer_id = ? AND order_date BETWEEN ? AND ?;`. Which of the following indexing strategies would be the most optimal for this query pattern?