Index-only scans

Indexes have already been described in Chapter 4, PostgreSQL Advanced Building Blocks. Simply speaking, indexes work like a glossary at the end of a book. When searching for a keyword in a book, to make it faster, you can look it up in the glossary and then go to the page specified. The glossary is alphabetically organized; that's why searching in it is fast.

Moreover, when you need to find out whether a keyword is present in the book, you don't have to go to the page  just looking in the glossary is enough.

PostgreSQL can do the same. If all the information that is needed for a query is contained in an index, the database won't perform the scan on the table data and will only use the index. This is called an index-only ...

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