What are the main recommendations and limitations when creating indexes in SQL systems?

Main recommendations and restrictions when creating indexes in SQL:

• Choose columns with high selectivity — indexes are effective when they help quickly filter a large number of rows.
• Do not index too many columns — excessive indexes slow down insert, update, and delete operations.
• Use composite indexes if you often filter by multiple columns simultaneously.
• Avoid indexing columns with low cardinality (e.g., boolean or with very limited set of values).
• Pay attention to data types — some types (e.g., TEXT or BLOB) are not always suitable for indexing.
• Regularly analyze and optimize indexes — remove unused ones and add necessary ones.

Example of creating an index:

CREATE INDEX idx_user_email ON users(email);

Indexes speed up SELECT queries but slow down INSERT/UPDATE/DELETE, so balance their number and types.