My question is at what database size, would the query performance be
affected?
We are actually planning to have a ArchivedStudent table(which has
exactly the same structure as that of Student table) so that when
student graduates, his record can be moved out of active Student table
to ArchivedStudent table. or should I just go for named scopes?
My question is at what database size, would the query performance be
affected?
Depends on how well, or poorly, crafted the query is? Do you have the
appropriate indexes defined to support your query? There are numerous
factors that will determine the actual performance. A classic "it
depends" question...