GET /mappings: Performance issues with direction=both

[stefandesu]

Now that direction=both is functionally fixed (see #219), it seems like using it is a far more expensive operation than combining direction=forward and direction=backward. Example:

https://coli-conc.gbv.de/api/mappings?properties=annotations&from=http://dewey.info/class/100/e23/|http://uri.gbv.de/terminology/sdnb/100|http://uri.gbv.de/terminology/bk/08.33|http://rvk.uni-regensburg.de/nt/CC%202500|http://rvk.uni-regensburg.de/nt/CI%201100|http://rvk.uni-regensburg.de/nt/CC%202600&toScheme=http://bartoc.org/en/node/486|http://bartoc.org/en/node/732|http://bartoc.org/en/node/1051|http://bartoc.org/en/node/335|http://bartoc.org/en/node/1031|http://bartoc.org/en/node/528|http://bartoc.org/en/node/313|http://bartoc.org/en/node/241|http://bartoc.org/en/node/20049|http://bartoc.org/en/node/430|http://bartoc.org/en/node/18785|http://bartoc.org/en/node/533|http://bartoc.org/en/node/20447|http://bartoc.org/en/node/20446&cardinality=1-to-1&direction=forward

takes 100-150ms (returning 22 results).

https://coli-conc.gbv.de/api/mappings?properties=annotations&from=http://dewey.info/class/100/e23/|http://uri.gbv.de/terminology/sdnb/100|http://uri.gbv.de/terminology/bk/08.33|http://rvk.uni-regensburg.de/nt/CC%202500|http://rvk.uni-regensburg.de/nt/CI%201100|http://rvk.uni-regensburg.de/nt/CC%202600&toScheme=http://bartoc.org/en/node/486|http://bartoc.org/en/node/732|http://bartoc.org/en/node/1051|http://bartoc.org/en/node/335|http://bartoc.org/en/node/1031|http://bartoc.org/en/node/528|http://bartoc.org/en/node/313|http://bartoc.org/en/node/241|http://bartoc.org/en/node/20049|http://bartoc.org/en/node/430|http://bartoc.org/en/node/18785|http://bartoc.org/en/node/533|http://bartoc.org/en/node/20447|http://bartoc.org/en/node/20446&cardinality=1-to-1&direction=backward

(the same request backwards) also takes 100-150ms (returning 56 results).

https://coli-conc.gbv.de/api/mappings?properties=annotations&from=http://dewey.info/class/100/e23/|http://uri.gbv.de/terminology/sdnb/100|http://uri.gbv.de/terminology/bk/08.33|http://rvk.uni-regensburg.de/nt/CC%202500|http://rvk.uni-regensburg.de/nt/CI%201100|http://rvk.uni-regensburg.de/nt/CC%202600&toScheme=http://bartoc.org/en/node/486|http://bartoc.org/en/node/732|http://bartoc.org/en/node/1051|http://bartoc.org/en/node/335|http://bartoc.org/en/node/1031|http://bartoc.org/en/node/528|http://bartoc.org/en/node/313|http://bartoc.org/en/node/241|http://bartoc.org/en/node/20049|http://bartoc.org/en/node/430|http://bartoc.org/en/node/18785|http://bartoc.org/en/node/533|http://bartoc.org/en/node/20447|http://bartoc.org/en/node/20446&cardinality=1-to-1&direction=both

(the same request with direction=both) takes ~23 seconds (returning the expected 78 results).

I would consider this a bug, as it shouldn't take more than the added times of the first two requests. I will need to investigate this.