Why can't indexes be added to all columns of a table?

Adding indexes to all columns of a table is not recommended for several reasons:

• Write overhead: when inserting, updating, or deleting each row, all indexes need to be updated, which slows down write operations.
• Increased data size: indexes occupy additional space on disk and in memory.
• Loss of efficiency: indexes on columns with low selectivity (few unique values) hardly speed up queries.
• Optimization complexity: too many indexes can confuse the query planner, leading it to choose an inefficient plan.

Therefore, indexes should be created consciously, based on actual queries and load.