Is it a good idea to index timestamp in postgresql?
For the query you give, the optimal general index would be on (my_timestamp, my_row_id, my_item_id), with the order of the last two columns possibly switched.  This will allow an index-only scan to be used which possibly doesn't visit the table at all, since all columns you reference would be available in the index.  How much of a performance improvement this would give you depends on how much of your table meets the timestamp WHERE condition, and how well-vacuumed you keep the table.  You might do even better with a partial index if the timestamp hardcoded into your query never changes, or changes in a very rote manner.


  Like store an extra column text 2018-08-17 from timestamp with timezone 2018-08-17 07:06:43.508778


There is little reason to think that this would be helpful in general, and certainly wouldn't help with the query you show (you would have to rewrite the query before it could use such an index, and then what happens if the timestamp is exactly '2018-09-13 00:00:00' ?)