Learn in the textbookE-R Diagrams, Key Design, and Normalization
DefinitionAn index (インデックス) is a lookup structure that maps a column's values to row locations, built to speed up searches.
ExplanationWithout an index, finding rows that meet a condition means scanning the table from top to bottom. With one, searches get faster, but every change to a row also requires updating the index, which adds work to each write and takes extra storage for the index itself.
AnalogyIt is like the index at the back of a thick book. You can jump from a word to its page right away, but whenever the text is revised, the index has to be revised too.
Memory hookIn English, index also names the forefinger (the index finger): picture it pointing straight at the row you want.