I wrote:
> If we wanted to keep the lengths in the same 32 bits they presumably
> occupy now, what about splitting 8/24 (=> 255 bytes for key, 24MB for
> value)?
Sigh, fingers faster than brain today. A 24-bit length field could
represent lengths up to 16MB, not 24MB. Still, it seems like a
reasonable maximum.
Or we could increase the size of hstore values so as to provide more
than 32 bits total for this, but that would presumably be pessimal for
all existing applications; there is evidently no one using more than
64K, or we'd have heard complaints before.
regards, tom lane