There are a few ways of achieving what you want.

• The one you are using (SELECT ... ORDER BY RAND() LIMIT 1) is easy and immediate. As you said, efficiency could be very poor when your table size increases.

  However, you can improve the execution speed if you only select the primary key column in your query.

  Note that this code can be used also if you want to get more than one random question at once.

  my $limit = 1;
  my $query = qq{SELECT id FROM table ORDER BY RAND() LIMIT $limit};
  my $ids = $dbh->selectcol_arrayref($query)
      or die "execution error ($DBI::errstr\n";
  $query = qq{SELECT question, answer FROM table
      WHERE id in (}
      . (join ",", @$ids)
      . " ) " ;
  [download]

  Selecting only the primary key will reduce the amount of data that the DBMS has to process. Therefore, selecting and sorting will be faster.

• A very fast alternative is available when your data doesn't change so often. If your data changes, say, once a day but you are querying around the clock, then you can make an auxiliary table each time you update or insert something.

  mysql> create table if not exists support_tbl
      -> (cnt int not null auto_increment primary key,
      -> id int not null);
  mysql> truncate support_tbl;
  mysql> insert into support_tbl (id) select id from table order by rand
  +();
  [download]

  Here you have a supporting table with primary keys in random order. Yopu can get a pageload of IDs very easily:

  my $cnt =1;
  my $ids = $dbh->selectcol_arrayref(qq{select id from 
      support_tbl where cnt = ?}, undef, $cnt);
  [download]

  For your next request, you will increase $cnt according to your needs.

• Another possibility is calculating the random value in Perl.

  # make sure our random value is not higher than
  # any value in the primary key
  my ($max) = @{$dbh->selectcol_arrayref(qq{select max(id) from table})}
  +;
  my $cnt = rand($max);
  my $query = qq{SELECT question, answer FROM table WHERE id = $cnt };
  [download]

  WARNING! Although this solution is fast, there is a catch. It will only work if your data does not have any gaps in your primary key column, i.e. if you deleted any row this method may not work as expected.

HTH

 _  _ _  _  
(_|| | |(_|><
 _|