Bit data type header reduction in some cases

Hi,

It's regarding a Todo item of "Bit data type header reduction" in some
cases. The header contains two parts. 1) The varlena header is
automatically converted to 1 byte header from 4 bytes in case of small
data. 2) The bit length header called "bit_len" to store the actual bit
length which is of 4 bytes in size. I want to reduce this bit_len size to 1
byte in some cases as similar to varlena header. With this change the size
of the column reduced by 3 bytes, thus shows very good decrease in disk
usage.

I am planning to the change it based on the total length of bits data. If
the number of bits is less than 0x7F then one byte header can be chosen
instead of 4 byte header. During reading of the data, the similar way it
can be calculated.

The problem I am thinking is, as it can cause problems to old databases
having bit columns when they
upgrade to a newer version without using pg_dumpall. Is there anyway to
handle this problem? Or Is there any better way to handle the problem
itself? please let me know your suggestions.

Regards,
Hari Babu
Fujitsu Australia