post__in for get_posts with a dynamic array

Here’s one way by using wpdb::get_col() and fetch the id column:

$pids = $wpdb->get_col( "SELECT id FROM my_table" );

We could then clean it by integer convert each id with:

$pids = wp_parse_id_list( $pids );

Just note the different max value for intval(), depending on 32 or 64 bits systems.