==== RELEVANT PAPER ====
[[http://www.google.com/url?sa=t&source=web&ct=res&cd=4&ved=0CCMQFjAD&url=http%3A%2F%2Fdspace.mit.edu%2Fbitstream%2Fhandle%2F1721.1%2F2295%2FSWP-3119-21583354-CIS-90-01.pdf%3Fsequence%3D1&rct=j&q=A+Polygen+Model+for+Heterogeneous+Database+Systems&ei=tXmZS-vJCYnINcuLwHo&usg=AFQjCNERfp-1tGeumT-prU-9ZWnSAYMmRQ&sig2=A0-UiNTG7khlOfLebhUZRA | A Polygen Model for Heterogeneous Database Systems: The Source Tagging Perspective]] (VLDB '90)

==== CONTRIBUTIONS ====

**Target Applications** 
          * Heterogeneous database systems that present a unified view of data compiled from various local sources.

**Goal:  ** 
          * Identify "source tags" i.e. local sources (databases) from which data originated. 
          * Identify "intermediate tags" i.e. all sources (databases) which were used to derive a given output.

**Contributions:** 
                 * Presents Polygen data model, built on top of the relational model.
                 * Describes the algebra underlying operations such as Select, Project, Join, Difference depicting how tags are propagated.
                 * Proposes a query translation mechanism for translating queries on Polygen tables to local tables. 

**Assumptions:** 
                 * Domain-mapping, attribute mapping, integration is done at priori. Considers only the problem of identifying tags.