String#[0] may return a byte, not a character.

sample.rb

require 'pg'

# $ docker run --rm -p 5432:5432 -e POSTGRES_PASSWORD=password -d postgres:9.6

conn = PG.connect(host: 'localhost', port: 5432, user: 'postgres', password: 'password', dbname: 'postgres')

conn.exec <<SQL
drop table if exists test_copy_to;
create table test_copy_to (str varchar);
insert into test_copy_to (str) values ('Ä');
SQL

puts ['str', 'str[0]', 'str.encoding'].join("\t")
puts ['---', '------', '------------'].join("\t")

dec = PG::TextDecoder::CopyRow.new
conn.copy_data('COPY test_copy_to TO STDOUT', dec) do
  while row = conn.get_copy_data
    # NOTE: I expect `row[0][0]` return `"Ä"`, but got `"\xC3"`.
    puts [row[0].inspect, row[0][0].inspect, row[0].encoding].join("\t")
    # NOTE: I learned this workaround.
    row[0].force_encoding('utf-8')
    puts [row[0].inspect, row[0][0].inspect, row[0].encoding].join("\t")
  end
end

dec = PG::TextDecoder::CopyRow.new(type_map: PG::TypeMapByColumn.new([PG::TextDecoder::Bytea.new]))
conn.copy_data('COPY (select * from test_copy_to) TO STDOUT', dec) do
  while row = conn.get_copy_data
    # NOTE: I learned this workaround.
    str = row[0].force_encoding('utf-8')
    puts [str.inspect, str[0].inspect, str.encoding].join("\t")
  end
end

str = conn.exec('select * from test_copy_to').getvalue(0, 0)
puts [str.inspect, str[0].inspect, str.encoding].join("\t")

__END__
ruby 2.5.3p105 (2018-10-18 revision 65156) [x86_64-darwin18]

str     str[0]  str.encoding
---     ------  ------------
"Ä"     "\xC3"  UTF-8
"Ä"     "Ä"     UTF-8
"Ä"     "Ä"     UTF-8
"Ä"     "Ä"     UTF-8