If you’ve queried a database, you’ve probably used an index, even if you didn't know it at the time. Database indexes help speed up read queries by creating ancillary data structures that make your scans faster.
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How do database indexes work?
If you’ve queried a database, you’ve probably used an index, even if you didn't know it at the time. Database indexes help speed up read queries by creating ancillary data structures that make your scans faster.
Full article