Inefficent subgraph retrievial

[thomaskrause]

When there are many nodes the subgraph retrieval gets inefficient since the "LIMIT 1" is not pushed down and we can get ugly cartesian products. We should refactor the whole subgraph processing code to produce cleaner and safer SQL.

This performance problem was found by Beni Ruef, who tried to execute the following query on their corpus

fonction="sujet" & type_pronom="personnel" &
categorie="clitique"&
norme="normé"&
degre_de_fusion="apostrophe"&
personne="1" &
nombre="sg" &
#1_=_#2 &
#2_=_#3 &
#3_=_#4 &
#4_=_#5 &
#5_=_#6 &
#6_=_#7