This is a *TEMPORAL WORKING* repository for ZHEAP upgrading, which developing branch is `REL_13_ZHEAP`.

The original ZHEAP repository doesn't consider version upgrades.
Therefore, upgrade work will be done in this temporary repository.

In the future, we plan to merge it with the ZHEAP repository.

The optimization code been added by the commit 695c597
to optimize this function.
However, due to this effect, when CREATE or ALTER TABLE statement is issued
in a transaction block and then ROLLBACK is issued, the postgres server is
crushed.
The reason is that ROLLBACK statement invokes HeapTupleSatisfiesMVCC() and
it calls HeapTupleHeaderGetCmin() and  GetRealCmin(), then Segmentation fault
is occurred.
To avoid this issue, we comment out this code.

…s bug.