Update: My mental block is cleared, and I have figured out most of my questions below, except for the part about initializing 'bar'

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Apologies in advance if this turns out to be too basic a question, but it has got me all stymied.

This is how I want my design to be -- the object constructor gets a couple of input values, and creates a fully populated object. There are three things to consider:

1. millions of objects are created, albeit not concurrently, during a program run
2. some aspects of initialization of all the objects, all millions of them, are controlled by values in a set of init config settings
3. a group of params in the objects are common to groups of objects, and since this group is a lot of values, they should be queried only once

Example pseudo code:

# In my long running program
my $dbname = get_db_name_from_some_config_file();
my $dbh = DBI->connect("dbi:SQLite:dbname=$dbname","","");
my $user_id = 1;
my $cfg = get_user_specific_config_values_stored_in_db($user_id);
for my $obj_id (0 .. 2 million) {
  my $obj = new Obj(dbh => $dbh, cfg => $cfg, obj_id => $obj_id);
  $obj->very_long_complicated_program();
}

# In a nearby module Obj.pm
sub new {
    my ($class, %args) = @_;
    my $dbh = $args{dbh}
    my $cfg = $args{cfg};
    my $obj_id = $args{obj_id};
  
    my $in = {
        dbh => $dbh,
        cfg => $cfg,
        foo => {},
        bar => {},
    };
    
    my $self = bless $in, $class;
    
    # foo is a result of a different query for every obj_id
    $in->{foo} = $self->foo($obj_id);

    # The following part is the only part I am still confused about.
    # Should I use something like Memoize here?
    #
    # bar changes only once per, say, every 5000 objects,
    # and the resulting query from $self->bar is a very 
    # large set (thousands of rows), so it makes sense to 
    # query the db only when bar changes. So, for obj_id 
    # 0 through 4999, bar remains the same, then for obj_id
    # 5000 through 9999 a new bar is returned, and so on
    $in->{bar} = $self->bar($obj_id);
    
    return $self;
}

sub dbh {
    my $class = shift;
    return $self->{dbh};
}

sub cfg {
    my $class = shift;
    return $self->{cfg};
}

sub foo {
    my ($class, $obj_id) = shift;

    my $cfg = $self->cfg;
    my $dbh = $self->dbh;
    my $foo = query_dbh_for_foo_execute($obj_id);
    $foo = change_parts_of_foo_based_on($cfg);
    return $foo;
}

sub bar {
    my ($class, $obj_id) = shift;

    my $dbh = $self->dbh;
    my $bar = query_dbh_for_bar_execute($obj_id);
    $bar = change_parts_of_bar_based_on($cfg);
    return $bar;
}
[download]

How do I go about implementing the above?

Update2: Another, possibly better way to ask my question -- how do I create an object a part of which (its data) is shared with other objects without requiring making a copy of that data?

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