The difference between MySQL full table scanning and indexing

Full table scanning means that all data in the table must be scanned before the data results can be displayed , Index scan is index , You only need to scan a portion of the data to get the results , Let's make an analogy , In Xinhua Dictionary , If there is no pinyin or stroke index , When we look for “ do ” This word should be looked up from the first page of the dictionary , All the way to Z The first part can be found , Even if you find it, you're not sure if there's still （ Suppose the dictionary is out of order ）, So we have to look back , Know that the original dictionary has been turned over , To be sure “ Oh , It turns out that the record just found is the desired result ”. Index scanning means that we know in advance “ do ” This word is in Pinyin Z Area , Then view according to the previous Directory "zuo" This pinyin is on that page , Then turn directly to that page and you can find the result we want , This can greatly reduce the query time