What is the primary purpose of adding relevant indexes when the join type is 'all'?

Adding relevant indexes when the join type is 'all' primarily serves to speed up query processing. When a database query involves joining large tables, particularly with the 'all' join type, which combines all records from both tables that meet the join condition, the performance can be significantly impacted.

Indexes allow the database management system to quickly locate and retrieve data without scanning the entire table. When appropriate indexes are in place, the database engine can use these indexes to narrow down the amount of data it needs to process during the join operation. This leads to faster retrieval times and overall improved efficiency in executing the query.

In contrast, minimizing database size, simplifying code maintenance, or enhancing visual representation do not directly relate to the operational performance aspect of executing queries. While these factors are important in database design and development, they do not address the immediate need for faster query execution, which is crucial when dealing with potentially large datasets associated with an 'all' join. Thus, the establishment of relevant indexes is fundamentally about optimizing performance in this context.