jsonb.h claims that the high order bit of a JEntry word is set on the
first element of a JEntry array. However, AFAICS, JBE_ISFIRST() is
not used anywhere, which is a good thing because it refers to a constant
JENTRY_ISFIRST that's not defined anywhere. Is this comment just a leftover
from a convention that's been dropped, or is it still implemented but not
via this macro?
If the bit is unused now, should we be worrying about reclaiming it for
better use? Like say allowing jsonb's to be larger than just a quarter
of the maximum datum size?
regards, tom lane
