Get all databases name present in Postgres Server in Drupal 7 Form API
One solution can be configurate the external database as a second database in settings.php.

$databases = array (
  'default' =&gt; 
  array (
    'default' =&gt; 
    array (
      'database' =&gt; 'standard_db',
      'username' =&gt; 'username',
      'password' =&gt; 'password',
      'host' =&gt; 'localhost',
      'port' =&gt; '',
      'driver' =&gt; 'postgres',
      'prefix' =&gt; '',
    ),
  ),
  'external' =&gt; 
  array (
    'default' =&gt; 
    array (
      'database' =&gt; 'external',
      'username' =&gt; 'username',
      'password' =&gt; 'password',
      'host' =&gt; 'db.external.com',
      'port' =&gt; '',
      'driver' =&gt; 'mysql',
      'prefix' =&gt; '',
    ),
  ),
);


Then before you do your database queries you need to set the active database. In this example, to switch to external you need to execute db_set_active('external');

When you want to switch back to the default database you execute db_set_active();

Example code to switch to and query the external database:

db_set_active('external');
$result = db_query('SELECT something FROM {sometable}');
while ($row = db_fetch_array($result)) {
  drupal_set_message($row['something']);
}

db_set_active();


And here are the queries that I have used for a similar project.

Getting the Databases:

SELECT datname 
FROM pg_database 
WHERE datallowconn = true AND NOT datname = 'postgres' AND NOT datname = 'template1'
ORDER BY datname


Getting the Schemas:

SELECT schema_name
FROM information_schema.schemata 
WHERE catalog_name = 'DATABASE_NAME' AND schema_name NOT LIKE 'pg_%' AND schema_name &lt;&gt; 'information_schema'
ORDER BY schema_name


Getting the Tables:

SELECT table_name
FROM information_schema.tables 
WHERE table_catalog = 'DATABASE_NAME' AND table_schema NOT LIKE 'pg_%' AND table_schema &lt;&gt; 'information_schema' and table_type = 'BASE TABLE'


Getting Fields and Field types

SELECT ordinal_position, column_name, 
CASE 
WHEN character_maximum_length &gt; 0 THEN concat(data_type, ' (',  character_maximum_length, ')')
ELSE data_type
END
AS data_type, is_nullable, column_default
FROM information_schema.columns
WHERE
table_schema = 'SCHEMA' and table_name = 'TABLE_NAME'


Reference


How to access the external database in drupal?
How to connect to multiple databases within Drupal