In RDBMS (Relational Database Management Systems), indexes are a special object that allow the user to quickly retrieve records from the database.
Typically, an index is implemented as a lookup table that has only two columns: the first column contains a copy of the primary or candidate key of a table; the second column contains a set of pointers for holding the address of the disk block where that specific key value is stored.
https://www.dbta.com/Columns/DBA-Corner/Top-10-Steps-to-Building-Useful-Database-Indexes-100498.aspx
It is common knowledge that judicious use of indexes can help SELECT queries execute significantly faster. This can tempt some database admins (DBAs) to try to milk as much performance gains as possible by adding indexes to every column that might possibly be included in a query.
The downside to adding indexes to a table is that they affect the performance of writes.
https://www.navicat.com/en/company/aboutus/blog/1764-the-downside-of-database-indexing.html
https://use-the-index-luke.com/