com.bigdata.bop.joinGraph.package.html Maven / Gradle / Ivy
Join graphs and query optimization.
This package provides support for join graphs, query optimization,
and generating full query plans from a join graph and the
evaluation order identified by a query optimizer.
Support is provided for both static query optimization, based on
an analysis of variable bindings and the cardinality of the
various access paths, and adaptive query optimization.
The adaptive query optimization technique is based on breadth
first cutoff sampling of join paths and dynamically selects a join
path providing efficient execution plan for the query. Fast and
accurate cardinality estimates are available based on range counts
are used to decide which joins to execute first. Without accurate
knowledge of detailed correlation in joins, estimation error
rapidly sets in as variable bindings are propagated. Adaptive
query optimization iteratively explores the join path identified
as having lowest cost for a common set of vertices. Join paths
whose costs are dominated by a better solution are eliminated.
Sampling of longer and longer join paths continues until one join
path is found to have the lowest cost, at which point it is
executed. This approach bears some similarity to the one
described in ROX : Run-time Optimization of XQueries , by Kader and Boncz
(2009) (presentation ), but there are numerous differences as well.