SELECT 
  tconnect_vendor_plans.allowed_vendor_subdomain 
FROM 
  tconnect_companies 
  INNER JOIN tconnect_vendor_plans ON tconnect_vendor_plans.plan_id = tconnect_companies.plan_id 
WHERE 
  tconnect_companies.company_id = 1

Query time 0.00122

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "tconnect_companies",
          "access_type": "const",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "4",
          "used_key_parts": ["company_id"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "tconnect_vendor_plans",
          "access_type": "const",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "4",
          "used_key_parts": ["plan_id"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100
        }
      }
    ]
  }
}