Use of soft references in java to build the cache and its limitations

Traversing a DAG

Database query optimization for joins on large tables ( join two tables with 1 million rows each ) in MYSQL

Particularly for MY SQL join between two tables takes much time like 5 min in case of very large tables .
If your requirement is to show data on UI  say in 5-20 sec max JOIN in query is useless .In that case we have implement join in memory using some caching techniques crudest may be use of soft cache .