How do relational databases maintain your set of results if another query has edited those rows that you were working on?
For instance if I select 100k rows and while I am still fetching those another query does an update on a row that hasn't been read yet I will not see the update.
Please lookup Multi Version Concurrency Control. Different databases have different approaches to managing this. For MySQL, InnoDB, you can try http://dev.mysql.com/doc/refman/5.0/en/innodb-multi-versioning.html. PostgreSQL - https://wiki.postgresql.org/wiki/MVCC. A great presentation here - http://momjian.us/main/writings/pgsql/mvcc.pdf. It is explained in stackoverflow in this thread Database: What is Multiversion Concurrency Control (MVCC) and who supports it?