To integrate many data sources we use a peer mediator framework where views defined in the peers are logically composed in terms of each other. A common approach to execute queries over mediators is to treat views in data sources as 'black boxes'. The mediators locally decompose queries into query fragments and submit them to the data sources for processing. Another approach, used in distributed DBMSs, is to treat the views as 'transparent boxes' by importing and fully expanding all views and

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... rge them with the query. The black box approach often leads to inefficient query plans. However, in a peer mediator framework full view expansion (VE) leads to prohibitively long query compilation times when many peers are involved. It also limits peer autonomy since peers must reveal their view definitions. We investigate in a peer mediator framework the tradeoffs between none, partial, and full VE in two different distributed view composition scenarios. We show that it is often favorable with respect to query execution and sometimes even with respect to query compilation time to expand those views having common hidden peer subviews. However, in other cases it is better to use the 'black box' approach, in particular when peer autonomy prohibits view importation. Based on this, a hybrid strategy for VE in peer mediators is proposed.