Retrieve index column
The documentation says:


  PRAGMA index_info(index-name);
  
  This pragma returns one row each column in the named index. The first column of the result is the rank of the column within the index. The second column of the result is the rank of the column within the table. The third column of output is the name of the column being indexed.


&gt; pragma index_info(comments_idx);
seqno       cid         name      
----------  ----------  ----------
0           0           books