First, make sure to properly index the tables you are joining. This will improve the performance of the join and reduce the amount of memory required.  Consider using a SELECT * FROM (SELECT * FROM table1 WHERE condition1 UNION SELECT * FROM table2 WHERE condition2) as subquery instead of a full outer join if possible. This will allow you to filter out unnecessary rows before the join, which can help reduce the amount of data being joined and therefore decrease memory usage.  Use LIMIT and OFFSET clauses to break up the full outer join into smaller chunks and process them one at a time. This can help prevent memory issues by limiting the amount of data being processed at any given time.  If you are using a database management system (DBMS) that supports partitioning, consider partitioning your tables by a common key to improve the performance of the full outer join.  If you are using a programming language such as Python or R, consider using a library that allows you to perform distributed full outer joins, which can help reduce memory usage by distributing the data across multiple machines.