title: sqlalchemy add index to existing sqlite3 database
body: <p>I have created a database with pandas :</p>
  
  <pre><code>import numpy as np                                                                                                                                                                                          
  import sqlite3                                                                                                                                                                                              
  import pandas as pd                                                                                                                                                                                         
  import sqlite3                                                                                                                                                                                              
  import sqlalchemy                                                                                                                                                                                           
  from sqlalchemy import create_engine                                                                                                                                                                        
  from sqlalchemy.orm import sessionmaker                                                                                                                                                                     
  
  df = pd.DataFrame(np.random.normal(0, 1, (10, 2)), columns=['A', 'B'])                                                                                                                                      
  
  path = 'sqlite:////home/username/Desktop/example.db'                                                                                                                                                        
  
  engine = create_engine(path, echo=False)                                                                                                                                                                    
  
  df.to_sql('flows', engine, if_exists='append', index=False)                                                                                                                                                 
  
  # This is only to show I am able to read the database                                                                                                                                                                                                            
  df_l = pd.read_sql("SELECT * FROM flows WHERE A&gt;0 AND B&lt;0", engine)                                                                                                                                         
  </code></pre>
  
  <p>Now I would like to add one or more  indexes to the database. 
  Is this case I would like to make first only the column <code>A</code> and then both the columns indices. </p>
  
  <p>How can I do that?</p>
  
  <p>If possible I would like a solution that uses only SqlAlchemy so that it is independent from the choice of the database. </p>