public function get_keys(string $table): ?array
  {
    if ( $tmp = $this->_get_cache($table) ){
      return [
        'keys' => $tmp['keys'],
        'cols' => $tmp['cols']
      ];
    }
    return null;
  }
public function get_keys(string $table): ?array
  {
    if ( $tmp = $this->_get_cache($table) ){
      return [
        'keys' => $tmp['keys'],
        'cols' => $tmp['cols']
      ];
    }
    return null;
  }
public function get_keys ( string $ table ) : ? array { if ( $ tmp = $ this -> _get_cache ( $ table ) ) { return [ 'keys' => $ tmp [ 'keys' ] , 'cols' => $ tmp [ 'cols' ] ] ; } return null ; }
Return the table's keys as an array indexed with the fields names.

```php
\bbn\x::dump($db->get_keys("table_users"));
/*
(array)[
"keys" => [
"PRIMARY" => [
"columns" => [
"id",
],
"ref_db" => null,
"ref_table" => null,
"ref_column" => null,
"unique" => 1,
],
"number" => [
"columns" => [
"number",
],
"ref_db" => null,
"ref_table" => null,
"ref_column" => null,
"unique" => 1,
],
],
"cols" => [
"id" => [
"PRIMARY",
],
"number" => [
"number",
],
],
]
```

@param string $table The table's name
@return null|array
Return the table s keys as an array indexed with the fields names .