  In the real world datasets (e.g.,DBpedia query log), queries built on
well-designed patterns containing only AND and OPT operators (for short,
WDAO-patterns) account for a large proportion among all SPARQL queries. In this
paper, we present a plugin-based framework for all SELECT queries built on
WDAO-patterns, named PIWD. The framework is based on a parse tree called
\emph{well-designed AND-OPT tree} (for short, WDAO-tree) whose leaves are basic
graph patterns (BGP) and inner nodes are the OPT operators. We prove that for
any WDAO-pattern, its parse tree can be equivalently transformed into a
WDAO-tree. Based on the proposed framework, we can employ any query engine to
evaluate BGP for evaluating queries built on WDAO-patterns in a convenient way.
Theoretically, we can reduce the query evaluation of WDAO-patterns to subgraph
homomorphism as well as BGP since the query evaluation of BGP is equivalent to
subgraph homomorphism. Finally, our preliminary experiments on gStore and
RDF-3X show that PIWD can answer all queries built on WDAO-patterns effectively
and efficiently.
