Heikki Linnakangas <[EMAIL PROTECTED]> writes:
> Saving 4 bytes per tuple with the phantom cid is nice, but saving 8 
> bytes (assuming we get rid of xvac in the future, or overlay it with 
> xmin for example) is even better.

xvac is not going away, so that argument is unconvincing, and I don't
believe you can avoid blowing out local memory if you have to remember
each tuple's cmin/cmax separately.  (Notice that Manfred gave up on his
patch for lack of a spill-to-disk mechanism.)

I'm also concerned about loss of debug traceability if these fields
disappear entirely from disk --- it's been handy more than once to be
able to tell where in a complex transaction something happened.

Lastly, at least on machines with 8-byte MAXALIGN, removing four more
bytes from heap headers would save nothing.  So I'm not excited about
going through enormous pushups to get rid of both fields, when a far
simpler and better-performing mechanism suffices to remove one.

                        regards, tom lane

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