Hi,
I just came across the following confusing thing.
zozo=# create table bit_test(i integer);
CREATE TABLE
zozo=# insert into bit_test values (1), (2), (3);
INSERT 0 3
zozo=# select i, i::bit(2), get_bit(i::bit(2), 1) as bit1,
get_bit(i::bit(2), 0) as bit0 from bit_test;i | i | bit1 | bit0
---+----+------+------1 | 01 | 1 | 02 | 10 | 0 | 13 | 11 | 1 | 1
(3 rows)
So, conversion from int to bitstring creates a readable bitstring, i.e.
the least significant bit is the rightmost one. But get_bit() on the same
bit string works in the opposite order. The only description about get_bit
I found in the 9.0beta docs are in
http://www.postgresql.org/docs/9.0/static/functions-binarystring.html#FUNCTIONS-BINARYSTRING-OTHER
-------------------------------------
Function Return Type Description Example Result
||
|get_bit|(string, offset) int Extract bit from string
get_bit(E'Th\\000omas'::bytea, 45) 1
-------------------------------------
||
and in
http://www.postgresql.org/docs/9.0/static/functions-bitstring.html
-------------------------------------
||The following functions work on bit strings as well as binary strings:
|get_bit|, |set_bit|.
-------------------------------------
Shouldn't it at least be documented in more depth? Say, get_bit(, N)
provides the Nth bit (0-based) counting from the leftmost bit?
I would certainly appreciate a warning spelled out about this
so if you convert a number to bitstring of length N and you want the
Mth bit (according to any programming language) then you need to use
get_bit(..., N-1-M).
Best regards,
Zoltán Böszörményi