Understanding a Slow Index in Database Queries

In database systems, indexes are used to make queries faster by helping the system find data quickly, similar to how a book index helps you find pages faster. However, the author says an index can still be “slow” when the database needs to do extra work after using the index. For example, if the query matches many rows or if the data is stored in different parts of memory, the database must open and read each row separately, which takes time. This idea connects to what I’ve learned in computer science and my experience with systems like SAP and MES, where performance depends not only on structure but also on how data is accessed. So, a “slow index” means the index works, but the process of fetching all the related data makes the query slower overall.

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