I wrote:
> David Steele <david@pgmasters.net> writes:
>> Maybe this would affect pg_controldata or other supporting utilities but
>> the server itself should not be affected since it also checks the
>> catalog version.
> Right, that version number exists mostly for the benefit of pg_controldata
> and allied utilities. You would get a CRC failure in any case when trying
> to go between inconsistent notions of what pg_control contains, but the
> version number is supposed to help you figure out why that happened.
> As things stand, though, it'd only mislead you.
Also notice that the server itself uses that heuristic to try to be
helpful about the reason for a pg_control read failure: it detects
PG_CONTROL_VERSION mismatch first, then checks CRC, then checks catversion
(see ReadControlFile(), which even goes to the trouble of trying to
diagnose endian issues this way). So the users-eye view of the problem
here is that starting a 9.5 or 9.6 server against a 9.4 pg_control file
will tell you that there's a checksum problem, not that the file is the
wrong version.
The value of getting this right was shown only today:
https://www.postgresql.org/message-id/6f746a21-ccfe-d736-57cf-04e3f87fcfa5@wintek.com
where we'd have had a heckuva harder time diagnosing what was up
with a less on-point error message.
Like Alvaro, I was on the fence about whether it is worth changing now;
but this consideration makes me think that it is.
regards, tom lane