If the lower left and upper right corners of a cube are the same, set a flag in the cube header, and only store one copy of the coordinates. That cuts the on-disk size into half for the common case that the cube datatype is used to represent points rather than boxes.
The new format is backwards-compatible with the old one, so pg_upgrade still works. However, to get the space savings, the data needs to be rewritten. A simple VACUUM FULL or REINDEX is not enough, as the old Datums will just be moved to the new heap/index as is. A pg_dump and reload, or something similar like casting to text and back, will do the trick.
This patch deliberately doesn't update all the alternative expected output files, as I don't have access to machines that produce those outputs. I'm not sure if they are still relevant, but if they are, the buildfarm will tell us and produce the diff required to fix it. If none of the buildfarm animals need them, they should be removed altogether.
Patch by Stas Kelvich.
08612f4 Extend cube on-disk format to pack points more tightly.
contrib/cube/cube.c | 483 +++++++++++++++++++++++---------------
contrib/cube/cubedata.h | 39 ++-
contrib/cube/cubeparse.y | 69 ++++--
contrib/cube/expected/cube_1.out | 281 ++++++++++++++++++++++
contrib/cube/expected/cube_2.out | 281 ++++++++++++++++++++++
contrib/cube/sql/cube.sql | 59 +++++
6 files changed, 990 insertions(+), 222 deletions(-)