Here is a quick picture about the impact of MySQL indexes misuse. At work, our developers made a new release of their search engine using [MySQL fulltext indexes][1], unfortunately they didn’t implement it correctly. The impact was a huge load on all our database servers. To find the trouble, I had to redirect the SQL search flow to a specific server and check for the slow queries then reproduce it with [EXPLAIN][2]. Don’t need a long time to find that the search query did an invalid usage of the fulltext index and the “Match / Against” syntax. In fact, the fulltext index was a multiple column fulltext index, in such case you have to specify ALL the column present in your index, else the index won’t be used by MySQL…

MySQL - Fulltext Indexes (MATH / AGAINST)

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