Star.Queries
OLAP queries typically include restrictions on multiple dimension tables that trigger
restrictions (via the foreign key relationships) on the (usually very large) fact
table. This is known as a star join (O??™Neil & Grafe, 1995). We use the term star
query to refer to flat SQL queries, defined over a single star schema, that include a
star join. Star queries represent the majority of OLAP queries. In particular, we are
42 Karayannidis, Tsois, & Sellis
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interested in ad hoc OLAP star queries. With the term ???ad hoc??? we refer to queries
that are not known in advance and therefore the administrator cannot optimize the
DBMS specifically for these.
In Figure 3 we depict the SQL query template for the ad hoc star queries considered.
The template defines the most complex query structure supported and uses abstract
terms that act as placeholders. Note that queries conforming to this template have
a structure that is a subset of the template in Figure 3 and instantiate all abstract
terms.
The terms D1, D2, ??¦, Dk are the dimension tables of the star join and LP1, LP2, ..,
LPk are the corresponding local predicates. Thus, the term LPi is a local predicate
on the dimension table Di.
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