SAP HANA Graphscript - degradation of performance on result with many rows - stored-procedures

Question

I've bumped into significant performance degradation of stored procedure when using HANA Graph script.
My task is following - I'm doing a BFS traverse on graph using standard BFS feature of HANA SP03. My graph is pretty dense and the result can easily go to a couple or several thousands of rows.
CREATE PROCEDURE "MY_PROC" (IN word VARCHAR(100), IN category VARCHAR(100), OUT res "RESULT" DEFAULT EMPTY)
LANGUAGE GRAPH READS SQL DATA AS
BEGIN
Graph g = Graph("SCHEMA1","MYGRAPH");
Multiset<Edge> filteredEdges = Multiset<Edge>(:g);
TRAVERSE BFS :g FROM Vertex(:g, :word)
ON VISIT EDGE (Edge e) {
Vertex sourceV = SOURCE(:e);
IF (:sourceV."WORD" != :word) {
filteredEdges = :filteredEdges UNION {:e};
}
};
--copy all results into output object
res = SELECT :e."TARGET", :e."CATEGORY_ID" FOREACH e IN :filteredEdges;
END;
I'm returning a TABLE type and use the following statement, pretty much the simplest thing possible as per tutorial:
It takes up to 10 seconds in my environment to prepare that result, which is obviously not acceptable. I've tested running time of other parts combined and it's up to tens of milliseconds. In case when result collection has only several hundreds of records running time became moderate - 100-200 miliseconds.
Is there another faster way of returning thousands of data from the graph script? I have a lot of liberty in my implementation, so I'll consider any approach that works. What I need in OUT parameter is a collection of some attributes of vertexes and of edges.
Thanks in advance

Answer 1

I think I got the answer, thanks to SAP HANA team guys.
There are several key ideas:
narrow initial graph to the smallest sub-graph possible using Subgraph
use custom BFS via NEIGHBORS instead of standard TRAVERSE BFS, just set limit big enough
use UNION ALL instead of UNION if logic allows - it's faster
so initial procedure transforms into something like this. Performance of the system soared to tens of milliseconds:
CREATE PROCEDURE "MY_PROC" (IN word VARCHAR(100), IN category VARCHAR(100), IN is_direct_category BOOLEAN, OUT res "TARGET_CATEGORY_RESULT" DEFAULT EMPTY)
LANGUAGE GRAPH READS SQL DATA AS
BEGIN
Graph = Graph("SCHEMA1","MYGRAPH");
Vertex startV = Vertex(:g_all, :word);
Multiset<Vertex> m_reachable= NEIGHBORS(:g_all, :startV, 0, 100);
Graph g = Subgraph(:g_all, :m_reachable);
if (:is_direct_category == TRUE) {
Multiset<Edge> properEdges = e in Edges(:g) where :e."CATEGORY_ID" == :category;
Graph res_g = Subgraph(:g, :properEdges);
Multiset<Edge> e_res = Edges(:res_g);
res = SELECT :hypoEdge."TARGET", :hypoEdge."CATEGORY_ID" FOREACH hypoEdge IN :e_res;
} else {
Multiset<Edge> e_res = Edges(:g);
res = SELECT :hypoEdge."TARGET", :hypoEdge."CATEGORY_ID" FOREACH hypoEdge IN :e_res;
}
END;