Most of the queries you’ll write will probably have WHERE clauses that compare values for equality, filter out ranges of rows, and so on. However, you might also need to perform keyword searches, which are based on relevance instead of comparing values to each other. Full-text search systems are designed for this purpose.

Full-text searches require a special query syntax. They can work with or without indexes, but indexes can speed up the matching. The indexes used for full-text searches have a special structure to help find documents that contain the desired keywords.

You might not know it, but you’re already familiar with at least one type of full-text search system: Internet search engines. Although they operate at a massive scale and don’t usually have a relational database for a backend, the principles are similar.