why do people use a database rather than the filesystem?

[1] SQL is a good language for manipulating structured data
    and for expressing predicates

[2] multiple indexes for performance

[3] SQL allows pushing some computation into server's address space

what interesting things can we do with histar?

[1] -> unclear, writing a SQL query processor seems difficult?

[2] -> explicit trust in indexes but maintain per-row data security

[3] -> pre-approve some aggregation or summarization operators while
       disallowing extracting original data.  e.g. allow extracting
       average salary or list of suggested purchases ala amazon..