afs commented on issue #1557:
URL: https://github.com/apache/jena/issues/1557#issuecomment-1264392984
Outstanding questions:
* Is the data in TDB? Or externally stored data?
* What is the actual slow part?
There are two potential costs in `GraphUnionRead`:
* it works in nodes when 2+ graphs
* it does general distinct that materialised nodes
When all the graphs are in a single TDB (either TDB1 or TDB2):
```SELECT (COUNT(*) AS ?c) FROM <x> FROM <y> { ?s ?p ?o }```
can be addressed by pushing down the work into TDB and not using the general
purpose `GraphUnionRead`. It will get the right answers. This isn't easy -
something will need to change because now we might multiple quad filtering in
one access. You'll need this anyway later on.
You may wish to review the requirements that Vilnis Termanis. Not the same
but related - maybe there is a single solution.
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