# bash 

file=/tmp/flatfile.dat  t=flattable  sep='|'

echo "head1|head2|head3
val1|val2|val3
val1|val4|val5
val6|val4|val5
val2|val7|val5
val3|val7|val3" > $file

# postgres extension 'file_fdw' must be installed (but haven't we all):
psql -qXa -c "create server if not exists fs foreign data wrapper file_fdw;" 

psql -qXa << SQL_PERL
drop foreign table if exists $t;
create foreign table $t ($( < $file perl -ne 'chomp; my $t = "'$t'";
print join("\n , ", map{$_ .= " text"} split(/['${sep}']/)); exit;'
))
server fs options(delimiter'$sep',format'csv',header'TRUE',filename'$file');
SQL_PERL


##</code><code>##

create server if not exists fs foreign data wrapper file_fdw;

drop foreign table if exists flattable;
create foreign table flattable (head1 text
 , head2 text
 , head3 text
)
server fs options(delimiter'|',format'csv',header'TRUE',filename'/tmp/flatfile.dat');

##</code><code>##


(select 'head1' "heads", array_agg(distinct head1 order by head1) "values" from flattable) union all 
(select 'head2' "heads", array_agg(distinct head2 order by head2) "values" from flattable) union all 
(select 'head3' "heads", array_agg(distinct head3 order by head3) "values" from flattable)

-- output:
--
--  heads |        values         
-- -------+-----------------------
--  head1 | {val1,val2,val3,val6}
--  head2 | {val2,val4,val7}
--  head3 | {val3,val5}
-- (3 rows)