Thanks Ashwin and Heikki for your responses. I've one more query here,
If BTree index is created on a zedstore table, the t_tid field of
Index tuple contains the physical tid that is not actually pointing to
the data block instead it contains something from which the logical
tid can be derived. So, when IndexScan is performed on a zedstore
table, it fetches the physical tid from the index page and derives the
logical tid out of it and then retrieves the data corresponding to
this logical tid from the zedstore table. For that, it kind of
performs SeqScan on the zedstore table for the given tid.
Nope, it won't perform seqscan. As zedstore is laid out as btree itself with logical TID as its key. It can quickly find which page the logical TID belongs to and only access that page. It doesn't need to perform the seqscan for the same. That's one of the rationals for laying out things in btree fashion to easily connect logical to physical world and not keep any external mapping.
AFAIU, the following user level query on zedstore table
select * from zed_tab where a = 3;
gets internally converted to
select * from zed_tab where tid = 3; -- assuming that index is created
on column 'a' and the logical tid associated with a = 3 is 3.
So, for this it will first only access the TID btree, find the leaf page with tid=3. Perform the visibility checks for the tuple and if tuple is visible, then only will fetch all the columns for that TID. Again using the btrees for those columns to only fetch leaf page for that logical tid.
Hope that helps to clarify the confusion.