SELECT id,
ST_AsGeoJSON(ST_Transform(ST_Simplify(way, $3*sqrt(ST_Area(ST_Envelope(way)))), 4326)) as data FROM
( SELECT id, ST_Multi(ST_Union(way)) AS way
FROM (
SELECT tags->'wikidata' AS id, (ST_Dump(way)).geom AS way
FROM $1~
WHERE tags ? 'wikidata' AND tags->'wikidata' IN ($2:csv)
) combq
GROUP BY id
) subq