Duplicate bindings on construct query

[coret]

Duplicate bindings on construct query

[rubensworks]

Strictly speaking, this is not really a bug, since CONSTRUCT queries return RDF graphs, which are sets of triples.
So if there are duplicate triples in the syntax, processors can remove duplicates following set semantics.

The reason why Comunica produces these duplicates is because CONSTRUCT queries are built on top of SELECT queries which instantiates triple templates, and may therefore produce duplicate triples.
Since Comunica produces results in a streaming manner, these duplicates are not removed.

That being said, I agree that it may sometimes be inconvenient to have these duplicates. So I'm converting this issue to a feature request where CONSTRUCT queries may optionally run in a DISTINCT-mode where duplicate triples will explicitly be removed, at the cost of an increase in memory usage for large results.

[karelklima]

I ran into this issue as well recently. My solution was to put the resulting triples to N3.Store and then iterate the data from there. Not efficient, but it got the job done.

[simonvbrae]

I have done a bit of work toward this on this branch.

Changes to packages/actor-init-query/lib/ActorInitQuery.ts were a first step to implementing deduplication using N3.Store.
Other changes add the necessary command line argument.

[github-actions]

Thanks for reporting!

[jacoscaz]

at the cost of an increase in memory usage for large results.

Just as a suggestion based on experience with jacoscaz/quadstore#155: when implementing the removal of duplicate triples consider giving users a way to set the maximum size of the set so that comunica may have a chance to throw an error and recover instead of potentially crashing the process by making it run out of memory.

[rubensworks]

A bounty has been placed on this issue via the Comunica Association (see original post).