Sorry, the correct version handling all cases. Seems to work when there are
gaps between segments and when segments are touching each other:
create or replace function st_makeShortestLine(g1 geometry, g2 geometry)
returns geometry as $$
with dist as (
select st_distance(st_endPoint($1), st_endPoint($2)) as dee,
st_distance(st_endPoint($1), st_startPoint($2)) as des,
st_distance(st_startPoint($1), st_endPoint($2)) as dse,
st_distance(st_startPoint($1), st_startPoint($2)) as dss
) select case when st_distance(st_startPoint($1), st_startPoint($2)) = 0
then st_makeline(st_reverse($1), $2)
when st_distance(st_startPoint($1), st_endPoint($2)) = 0 then
st_makeline($2, $1)
when st_distance(st_endPoint($1), st_endPoint($2)) = 0 then
st_makeline($1, st_reverse($2))
when st_distance(st_endPoint($1), st_startPoint($2)) = 0 then
st_makeline($1, $2)
when dee < des and dee < dse and dee < dss then st_makeline($1,
st_reverse($2))
when des < dee and des < dse and des < dss then st_makeline($1, $2)
when dss < dee and dss < dse and dss < des then
st_makeline(st_reverse($1), $2)
else st_makeline($2, $1) end
from dist;
$$ language SQL;
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