Prøv:
create or replace function get_query_tables(p_query text) returns text[] language plpgsql as $$
declare
x xml;
begin
execute 'explain (format xml) ' || p_query into x;
return xpath('//explain:Relation-Name/text()', x, array[array['explain', 'http://www.postgresql.org/2009/explain']])::text[];
end $$;
select get_query_tables('your query here');