An index contains keys built from one or more columns in the table, or view, and pointers that map to the storage location of the specified data.

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SQL Server has NEVER put an index on a foreign key column… Indexes are used to make the lookup (in a primary or unique key) for a duplicate value FAST.

The absolute worst thing about B+Trees is their name. It is like someone maliciously considered all the possible names and then selected the most confusing one.