Zdenek Kotala wrote:
By my opinion first place where tuple should be placed is:

MAXALIGN(SizeOfPageHeaderData + sizeof(ItemIdData))

Yeah, but we don't enforce that directly. We enforce it by MAXALIGNing size in PageAddItem, and with the rule that special-size is MAXALIGNed.

To put it another way, it's possible that there's an unaligned amount of free space on a page, as returned by PageGetExactFreeSpace. But since item size is always MAXALIGNed, it's impossible to use it all.

Come to think of it, why do we require MAXALIGN alignment of tuples? I must be missing something, but AFAICS the widest fields in HeapTupleHeaderData are 4-bytes wide, so it should be possible to get away with 4-byte alignment.

  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com

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