We remove the third wait state completely from the concurrent index build. When we mark the index as ready, we also mark “indcheckxmin” to true which essentially enforces Postgres to not use this index for older snapshots.
I think there is a problem in the proposed solution. When phase 3 is reached, the index is valid. But it might not contain tuples deleted just before the reference snapshot was taken. Hence, we wait for those transactions that might have older snapshot. The TransactionXmin of these transactions can be greater than the xmin of the pg_index entry for this index.
Instead of waiting in the third phase, if we just set indcheckxmin as true, the above transactions will be able to use the index which is wrong. (because they won't find the recently deleted tuples from the index that are still live according to their snapshots)