1. Slower Write Operations

• INSERT, UPDATE, DELETE operations become slower.
• Why? Every time you modify the table, the database has to:
  • Update the table and
  • Update all related indexes.

Example:

INSERT INTO Customers VALUES (100, 'Dave');

If there’s an index on customer_id, the database must:

• Insert the row.
• Also insert the new key into the index tree (e.g., B-tree) in the correct position.

For large tables with many indexes — writing gets heavier.

2. Increased Storage Usage

• Indexes consume disk space.
• More indexes = more disk space used.
• In large tables, indexes can be as big as the data itself.

Example:

• A table of 10 million rows with 3 indexes might use 3–5x more storage than the raw table.

3. Maintenance Overhead

• If the data changes frequently (lots of updates/deletes), the indexes can become fragmented.
• Fragmentation slows down queries over time.
• Requires rebuilding or reorganizing indexes periodically (REBUILD INDEX, OPTIMIZE TABLE).

4. Choosing Wrong Index Can Hurt Performance

• If the wrong column(s) are indexed or the wrong type of index is used:
  • The optimizer might pick a bad execution plan.
  • You might slow down queries rather than speeding them up.

Example:

• Index on last_name but you search by email — index won’t help.

5. Complexity

• More indexes = more complexity for:
  • Database administrators (DBAs).
  • Developers (need to understand which queries use which indexes).
• Harder to tune performance manually when there are too many indexes.

6. Index Overhead on Bulk Operations

• Bulk loads (like BULK INSERT) or mass updates can be painfully slow if indexes exist.
• Often people drop indexes, load data, and then recreate indexes afterward.

Quick Summary

Disadvantage	Impact
Slows down INSERT/UPDATE/DELETE	Index must be updated on every data change.
Extra storage usage	Indexes take disk space.
Fragmentation	Indexes degrade over time, need maintenance.
Bad index design hurts queries	Poor indexes can slow down, not speed up.
More complexity	Managing many indexes becomes harder.
Slower bulk operations	Large inserts/updates slow down significantly.

Golden Rule

👉 Index what you query, not what you insert!

• Only create indexes that support your frequent queries.
• Be careful not to “over-index” — balance is key!