Severe Performance Decrease in 1.3.0 when using set_single_row_mode

[jeremyevans]

Reproducer:

gem 'pg', ARGV[0] if ARGV[0]
require 'pg'
conn = PG::Connection.new(user: "sequel_test")
conn.exec("DROP TABLE IF EXISTS accounts;")
conn.exec("CREATE TABLE accounts (id integer default 0);")

conn.exec("INSERT INTO accounts DEFAULT VALUES;")
18.times do
  conn.exec("INSERT INTO accounts SELECT * FROM accounts;")
end

conn.send_query("SELECT * FROM accounts")
conn.set_single_row_mode
conn.block

t = Time.now
nil while conn.get_result
puts "#{Time.now - t} seconds"

Results (ruby is ruby 3.1.1p18, but this same issue occurs for all Ruby versions tested):

$ ruby t2.rb 1.1.4
4.499579633 seconds
$ ruby t2.rb 1.2.3
4.440106149 seconds
$ ruby t2.rb 1.3.0
72.99373833 seconds
$ ruby t2.rb 1.3.1
72.075049592 seconds

Originally discovered by @janko and reported at jeremyevans/sequel#1846, but determined to be a ruby-pg issue and not a Sequel issue. You can probably mitigate this issue by using stream_each, but currently that cannot be used by sequel_pg. I'll send a pull request for the changes needed to make it usable by sequel_pg.