Arbitrary tuple size

Well,

doing arbitrary tuple size should be as generic as possible.
Thus I think the best place to do it is down in the heapam
routines (heap_fetch(), heap_getnext(), heap_insert(), ...).
I'm not 100% sure but nothing should access a heap relation
going around them. Anyway, if there are places, then it's
time to clean them up.

What about adding one more ItemPointerData to the tuple
header which holds the ctid of a DATA continuation tuple. If
a tuple doesn't fit into one block, this will tell where to
get the next chunk of tuple data building a chain until an
invalid ctid is found. The continuation tuples can have a
negative t_natts to be easily identified and ignored by
scanning routines.

By doing it this way we could also sqeeze out some currently
wasted space. All tuples that get inserted/updated are added
to the end of the relation. If a tuple currently doesn't fit
into the freespace of the actual last block, that freespace
is wasted and the tuple is placed into a new allocated block
at the end. So if there is 5K freespace and another 5.5K
tuple is added, the relation grows effectively by 10.5K!

I'm not sure how to handle this with vacuum, but I believe
Vadim is able to put some well placed goto's that make it.

Jan

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