Alvaro Herrera <[EMAIL PROTECTED]> writes:
> On Fri, Jul 08, 2005 at 09:38:07AM +0900, ITAGAKI Takahiro wrote:
>> Avoiding overheads, can XIDs/CIDs be different bit length? For example,
>> can XIDs/CIDs be changed to 48/16-bit or 40/24-bit?

> Not unless you change the definition of HeapTupleFields
> (src/include/access/htup.h).  Alignment concerns would probably bite you
> if you changed it, anyway.

I don't think we could feasibly reduce the width of CID anyway; we've
already seen a few complaints of people overrunning 2^32 commands per
transaction, and surely this is a bigger rather than a lesser concern
if you are thinking of large databases.

It probably would be possible to keep CID at 32 bits and lay out the
HeapTupleHeader so that you only pay for three, not four, 64-bit
fields ... but that's still twelve bytes added per tuple.

                        regards, tom lane

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