Is the system column "ctid" legitimate for identifying rows to delete? I have a table with hundreds of millions of rows that I need to delete data from.

The existing indexes are the most efficient.

I can however use the existing indexes to find the rows to delete by using the ctid values:

DELETE FROM calendar_event WHERE ctid IN
(SELECT ctid FROM calendar_event WHERE user_id = 5 LIMIT 100 FOR UPDATE)


What are the risks of relying on the ctid in this case? My worst case scenario is deleting the wrong the row.
The ROW SHARE lock taken by FOR UPDATE prevents concurrent write access that would change the physical location of the row. The manual:


  This prevents them from being locked, modified or deleted by other
  transactions until the current transaction ends. That is, other
  transactions that attempt UPDATE, DELETE, SELECT FOR UPDATE, SELECT FOR NO KEY UPDATE, SELECT FOR SHARE or SELECT FOR KEY SHARE of these
  rows will be blocked until the current transaction ends;


So the ctid should be stable for the duration of the command (or the transaction, even) unless you alter the row within the same transaction yourself. ctid is still a system column for internal use and the project will not offer any guarantees. If you have any unique (combination of) column(s) (including the PK) use that instead of the ctid.

However, I would use a CTE to materialize the selection and avoid unexpected results.

And without ORDER BY you select arbitrary rows for deletion. You might as well add SKIP LOCKED to minimize lock contention with concurrent transactions:

WITH cte AS (
   SELECT ctid
   FROM   calendar_event
   WHERE  user_id = 5
   LIMIT  100
   FOR    UPDATE SKIP LOCKED
   )
DELETE FROM calendar_event WHERE ctid IN (TABLE cte);


Related, with explanation for both considerations:


Postgres UPDATE ... LIMIT 1


About ctid:


How do I decompose ctid into page and row numbers?