Re: pg_upgarde: The control data check for equal max_index_keys in old and new version

neha khatri <nehakhatri5@gmail.com> writes:
> I have recently started working on Postgresql and I was looking in the the
> pg_upgrade code. pg_upgrade checks index control data(max_index_keys) to be
> equal in the old and new version.

> I would like to understand why the number of index columns should be equal
> in new and old versions.

Because that changes the size of index tuple headers, see
src/include/access/itup.h

> Can the number of maximum index columns not be
> changed from one release to another release.

Sure, as long as you don't care about on-disk database compatibility.
The checks associated with pg_control are meant to ensure exactly
that, though.

            regards, tom lane