What criteria are used to determine which database columns should have indexes?

When choosing columns for creating indexes in a database, the following criteria are considered:

• Frequency of use in WHERE conditions: columns that are frequently filtered are good candidates for indexing.
• Use in JOINs: columns involved in table joins are often indexed to speed up the process.
• Columns used for sorting (ORDER BY) or grouping (GROUP BY): indexes help accelerate these operations.
• Data cardinality: it is better to index columns with high uniqueness of values, as the index will be more effective.
• Table size: in very small tables, indexes may not provide a performance gain.
• Frequency of data updates: if a column is frequently updated, the index may slow down write operations.

Thus, indexes are created where they provide a speed advantage in reading, without significantly degrading write performance.