Alternative variable length structure

Hi Hackers,

PostgreSQL can treat variable-length data flexibly, but therefore
it consumes more spaces if we store short data. Headers of
variable-length types use 4 bytes regardless of the data length.

My idea is to change the header itself to variable-length.
In order to reduce the size of short data, I wrote a patch to encode
lengths into the first several bits of structure. Also, the alignments
of the types were changed to 'char' from 'int'.

I know my patch is still insufficient, for example, the types cannot
be TOASTed. But I guess this compression works well for short text.

I'll appreciate any comments.
thanks.

---- the result of patch ----

# create table txttbl (v1 text, v2 text, v3 text, v4 text);
# create table strtbl (v1 string, v2 string, v3 string, v4 string);

# insert into txttbl values('A', 'B', 'C', 'D');
# insert into strtbl values('A', 'B', 'C', 'D');

# select * from pgstattuple('txttbl');
-[ RECORD 1 ]------+------
table_len | 8192
tuple_count | 1
tuple_len | 57 <-- 28 + (5+3) + (5+3) + (5+3) + (5)
...

# select * from pgstattuple('strtbl');
-[ RECORD 1 ]------+------
table_len | 8192
tuple_count | 1
tuple_len | 36 <-- 28 + 2 + 2 + 2 + 2
...

---
ITAGAKI Takahiro
NTT Cyber Space Laboratories