Postgres 12

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select version();create table if not exists lines(
id serial,
colour varchar,
geom geometry(linestring,4326)
);
create table if not exists points(
id serial,
colour varchar,
geom geometry(point,4326)
);

insert into lines(colour, geom)
VALUES
('blue', st_setsrid(st_makeline(st_makepoint(1,1), st_makepoint(10,1)),4326)),
('yellow', st_setsrid(st_makeline(st_makepoint(1,1), st_makepoint(3.5,1)),4326)),
('yellow', st_setsrid(st_makeline(st_makepoint(1,1), st_makepoint(3.5,1)),4326)),
('yellow', st_setsrid(st_makeline(st_makepoint(1.0,1), st_makepoint(5.5,1)),4326)),
('yellow', st_setsrid(st_makeline(st_makepoint(5.5,1), st_makepoint(8.5,1)),4326)),
('yellow', st_setsrid(st_makeline(st_makepoint(7,1), st_makepoint(10,1)),4326));
insert into points(colour, geom)
VALUES
('blue',st_setsrid(st_makepoint(3,1),4326)),
('blue',st_setsrid(st_makepoint(6,1),4326)),
('blue',st_setsrid(st_makepoint(9,1),4326)),
('yellow',st_setsrid(st_makepoint(3,1),4326)),
('yellow',st_setsrid(st_makepoint(6,1),4326)),
('yellow',st_setsrid(st_makepoint(9,1),4326));

with line_shenanigans as(
with knife as(
    select st_union(geom) as geom, colour
    from points
    group by colour),
union_lines as(
	select st_union(geom) as u_geom, st_collect(geom) as c_geom, colour
    from lines
	group by colour)
	
select union_lines.colour,
(st_dump(st_split(st_linemerge(union_lines.u_geom),knife.geom))).geom as ul_geom,
(st_dump(st_split(st_linemerge(union_lines.c_geom),knife.geom))).geom as cl_geom,
(st_dump(st_split(union_lines.u_geom,knife.geom))).geom as u_geom,
(st_dump(st_split(union_lines.c_geom,knife.geom))).geom as c_geom
from union_lines, knife
where union_lines.colour=knife.colour
)
select colour,
sum(st_length(ul_geom)) as union_lm_length, 
count(ul_geom) as union_lm_segments,
sum(st_length(cl_geom)) as collect_lm_length,
count(cl_geom) as collect_lm_segments,
sum(st_length(u_geom)) as union_length, 
count(u_geom) as union_segments,
sum(st_length(c_geom)) as collect_length, 
count(c_geom) as collect_segments
from line_shenanigans
group by colour

version
PostgreSQL 12.9 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-4), 64-bit

6 rows affected

6 rows affected

colour	union_lm_length	union_lm_segments	collect_lm_length	collect_lm_segments	union_length	union_segments	collect_length	collect_segments
blue	9	4	9	4	9	4	9	4
yellow	9	4	15.5	7	9	8	15.5	10