Postgres 12

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
CREATE TABLE element (id int, geo geometry);
INSERT INTO element VALUES (1,ST_Buffer(ST_MakePoint(-71.10, 42.31), 6));

UPDATE element
SET geo =   
  ST_ForceCollection(
    ST_Union(geo, ST_Buffer(ST_MakePoint(-80.10, 45.31), 3) ))
WHERE id = 1;
	  
SELECT ST_AsText(geo) FROM element;	  
1 rows affected

1 rows affected

st_astext

GEOMETRYCOLLECTION(POLYGON((-65.2152883175806 41.1394580679032,-65.5567228049323 40.0138994058095,-66.1111823261847 38.9765786018824,-66.8573593128807 38.0673593128807,-67.7665786018824 37.3211823261847,-68.8038994058094 36.7667228049323,-69.9294580679032 36.4252883175806,-71.1 36.31,-72.2705419320967 36.4252883175806,-73.3961005941905 36.7667228049323,-74.4334213981176 37.3211823261847,-75.3426406871193 38.0673593128807,-76.0888176738153 38.9765786018824,-76.6432771950677 40.0138994058094,-76.9847116824194 41.1394580679032,-77.1 42.31,-76.9847116824194 43.4805419320967,-76.6432771950677 44.6061005941905,-76.0888176738153 45.6434213981176,-75.3426406871193 46.5526406871193,-74.4334213981176 47.2988176738153,-73.3961005941906 47.8532771950677,-72.2705419320968 48.1947116824194,-71.1 48.31,-69.9294580679033 48.1947116824194,-68.8038994058095 47.8532771950677,-67.7665786018824 47.2988176738153,-66.8573593128807 46.5526406871193,-66.1111823261847 45.6434213981176,-65.5567228049323 44.6061005941906,-65.2152883175806 43.4805419320968,-65.1 42.31,-65.2152883175806 41.1394580679032)),POLYGON((-77.1576441587903 44.7247290339516,-77.3283614024661 44.1619497029047,-77.6055911630924 43.6432893009412,-77.9786796564403 43.1886796564404,-78.4332893009412 42.8155911630924,-78.9519497029047 42.5383614024661,-79.5147290339516 42.3676441587903,-80.1 42.31,-80.6852709660484 42.3676441587903,-81.2480502970953 42.5383614024661,-81.7667106990588 42.8155911630924,-82.2213203435596 43.1886796564404,-82.5944088369076 43.6432893009412,-82.8716385975338 44.1619497029047,-83.0423558412097 44.7247290339516,-83.1 45.31,-83.0423558412097 45.8952709660484,-82.8716385975339 46.4580502970953,-82.5944088369076 46.9767106990588,-82.2213203435596 47.4313203435596,-81.7667106990588 47.8044088369076,-81.2480502970953 48.0816385975339,-80.6852709660484 48.2523558412097,-80.1 48.31,-79.5147290339516 48.2523558412097,-78.9519497029047 48.0816385975339,-78.4332893009412 47.8044088369076,-77.9786796564404 47.4313203435597,-77.6055911630924 46.9767106990588,-77.3283614024661 46.4580502970953,-77.1576441587903 45.8952709660484,-77.1 45.31,-77.1576441587903 44.7247290339516)))

WITH j (geom) AS (
 VALUES (ST_Buffer(ST_MakePoint(-71.10, 42.31), 6)),
        (ST_Buffer(ST_MakePoint(-80.10, 45.31), 3))  
)
SELECT ST_AsText(ST_ForceCollection(ST_Union(geom))) 
FROM j; 

st_astext

GEOMETRYCOLLECTION(POLYGON((-65.2152883175806 41.1394580679032,-65.5567228049323 40.0138994058095,-66.1111823261847 38.9765786018824,-66.8573593128807 38.0673593128807,-67.7665786018824 37.3211823261847,-68.8038994058094 36.7667228049323,-69.9294580679032 36.4252883175806,-71.1 36.31,-72.2705419320967 36.4252883175806,-73.3961005941905 36.7667228049323,-74.4334213981176 37.3211823261847,-75.3426406871193 38.0673593128807,-76.0888176738153 38.9765786018824,-76.6432771950677 40.0138994058094,-76.9847116824194 41.1394580679032,-77.1 42.31,-76.9847116824194 43.4805419320967,-76.6432771950677 44.6061005941905,-76.0888176738153 45.6434213981176,-75.3426406871193 46.5526406871193,-74.4334213981176 47.2988176738153,-73.3961005941906 47.8532771950677,-72.2705419320968 48.1947116824194,-71.1 48.31,-69.9294580679033 48.1947116824194,-68.8038994058095 47.8532771950677,-67.7665786018824 47.2988176738153,-66.8573593128807 46.5526406871193,-66.1111823261847 45.6434213981176,-65.5567228049323 44.6061005941906,-65.2152883175806 43.4805419320968,-65.1 42.31,-65.2152883175806 41.1394580679032)),POLYGON((-77.1576441587903 44.7247290339516,-77.3283614024661 44.1619497029047,-77.6055911630924 43.6432893009412,-77.9786796564403 43.1886796564404,-78.4332893009412 42.8155911630924,-78.9519497029047 42.5383614024661,-79.5147290339516 42.3676441587903,-80.1 42.31,-80.6852709660484 42.3676441587903,-81.2480502970953 42.5383614024661,-81.7667106990588 42.8155911630924,-82.2213203435596 43.1886796564404,-82.5944088369076 43.6432893009412,-82.8716385975338 44.1619497029047,-83.0423558412097 44.7247290339516,-83.1 45.31,-83.0423558412097 45.8952709660484,-82.8716385975339 46.4580502970953,-82.5944088369076 46.9767106990588,-82.2213203435596 47.4313203435596,-81.7667106990588 47.8044088369076,-81.2480502970953 48.0816385975339,-80.6852709660484 48.2523558412097,-80.1 48.31,-79.5147290339516 48.2523558412097,-78.9519497029047 48.0816385975339,-78.4332893009412 47.8044088369076,-77.9786796564404 47.4313203435597,-77.6055911630924 46.9767106990588,-77.3283614024661 46.4580502970953,-77.1576441587903 45.8952709660484,-77.1 45.31,-77.1576441587903 44.7247290339516)))

SELECT 
 ST_AsText(
  ST_ForceCollection(
    ST_Union(
      ST_Buffer(ST_MakePoint(-71.10, 42.31), 6),
      ST_Buffer(ST_MakePoint(-80.10, 45.31), 3))))
 
st_astext

GEOMETRYCOLLECTION(POLYGON((-65.2152883175806 41.1394580679032,-65.5567228049323 40.0138994058095,-66.1111823261847 38.9765786018824,-66.8573593128807 38.0673593128807,-67.7665786018824 37.3211823261847,-68.8038994058094 36.7667228049323,-69.9294580679032 36.4252883175806,-71.1 36.31,-72.2705419320967 36.4252883175806,-73.3961005941905 36.7667228049323,-74.4334213981176 37.3211823261847,-75.3426406871193 38.0673593128807,-76.0888176738153 38.9765786018824,-76.6432771950677 40.0138994058094,-76.9847116824194 41.1394580679032,-77.1 42.31,-76.9847116824194 43.4805419320967,-76.6432771950677 44.6061005941905,-76.0888176738153 45.6434213981176,-75.3426406871193 46.5526406871193,-74.4334213981176 47.2988176738153,-73.3961005941906 47.8532771950677,-72.2705419320968 48.1947116824194,-71.1 48.31,-69.9294580679033 48.1947116824194,-68.8038994058095 47.8532771950677,-67.7665786018824 47.2988176738153,-66.8573593128807 46.5526406871193,-66.1111823261847 45.6434213981176,-65.5567228049323 44.6061005941906,-65.2152883175806 43.4805419320968,-65.1 42.31,-65.2152883175806 41.1394580679032)),POLYGON((-77.1576441587903 44.7247290339516,-77.3283614024661 44.1619497029047,-77.6055911630924 43.6432893009412,-77.9786796564403 43.1886796564404,-78.4332893009412 42.8155911630924,-78.9519497029047 42.5383614024661,-79.5147290339516 42.3676441587903,-80.1 42.31,-80.6852709660484 42.3676441587903,-81.2480502970953 42.5383614024661,-81.7667106990588 42.8155911630924,-82.2213203435596 43.1886796564404,-82.5944088369076 43.6432893009412,-82.8716385975338 44.1619497029047,-83.0423558412097 44.7247290339516,-83.1 45.31,-83.0423558412097 45.8952709660484,-82.8716385975339 46.4580502970953,-82.5944088369076 46.9767106990588,-82.2213203435596 47.4313203435596,-81.7667106990588 47.8044088369076,-81.2480502970953 48.0816385975339,-80.6852709660484 48.2523558412097,-80.1 48.31,-79.5147290339516 48.2523558412097,-78.9519497029047 48.0816385975339,-78.4332893009412 47.8044088369076,-77.9786796564404 47.4313203435597,-77.6055911630924 46.9767106990588,-77.3283614024661 46.4580502970953,-77.1576441587903 45.8952709660484,-77.1 45.31,-77.1576441587903 44.7247290339516)))