What reasons can cause a database index not to be used for query execution?

An index in a database may not be used to execute a query for several reasons:

• Low selectivity of the index: if the index covers columns with a large number of repeating values, the optimizer may decide that a full table scan is faster.

• Use of functions or operations on an indexable column: for example, calling a function on a column (WHERE LOWER(name) = 'john') can prevent the use of the index.

• Query with conditions not covered by the index: if the query uses columns not included in the index, or conditions that do not match the index structure.

• Small table size: for small tables, the optimizer may choose a full scan because it is faster.

• Lack of statistics or outdated statistics: the optimizer makes decisions based on statistics; if they are outdated, the index may not be used.

• Using the LIKE operator with a pattern starting with a wildcard (LIKE '%abc'), which prevents the use of the index.

• Queries with OR that cannot be efficiently covered by the index.

Understanding these reasons helps optimize queries and index structures.