Is it possible to perform a bitwise group function? - sql

Question

I have a field in a table which contains bitwise flags. Let's say for the sake of example there are three flags: 4 => read, 2 => write, 1 => execute and the table looks like this*:
user_id | file | permissions
-----------+--------+---------------
1 | a.txt | 6 ( <-- 6 = 4 + 2 = read + write)
1 | b.txt | 4 ( <-- 4 = 4 = read)
2 | a.txt | 4
2 | c.exe | 1 ( <-- 1 = execute)
I'm interested to find all users who have a particular flag set (eg: write) on ANY record. To do this in one query, I figured that if you OR'd all the user's permissions together you'd get a single value which is the "sum total" of their permissions:
user_id | all_perms
-----------+-------------
1 | 6 (<-- 6 | 4 = 6)
2 | 5 (<-- 4 | 1 = 5)
*My actual table isn't to do with files or file permissions, 'tis but an example
Is there a way I could perform this in one statement? The way I see it, it's very similar to a normal aggregate function with GROUP BY:
SELECT user_id, SUM(permissions) as all_perms
FROM permissions
GROUP BY user_id
...but obviously, some magical "bitwise-or" function instead of SUM. Anyone know of anything like that?
(And for bonus points, does it work in oracle?)

Answer 1

MySQL:
SELECT user_id, BIT_OR(permissions) as all_perms
FROM permissions
GROUP BY user_id

Answer 2

Ah, another one of those questions where I find the answer 5 minutes after asking... Accepted answer will go to the MySQL implementation though...
Here's how to do it with Oracle, as I discovered on Radino's blog
You create an object...
CREATE OR REPLACE TYPE bitor_impl AS OBJECT
(
bitor NUMBER,
STATIC FUNCTION ODCIAggregateInitialize(ctx IN OUT bitor_impl) RETURN NUMBER,
MEMBER FUNCTION ODCIAggregateIterate(SELF IN OUT bitor_impl,
VALUE IN NUMBER) RETURN NUMBER,
MEMBER FUNCTION ODCIAggregateMerge(SELF IN OUT bitor_impl,
ctx2 IN bitor_impl) RETURN NUMBER,
MEMBER FUNCTION ODCIAggregateTerminate(SELF IN OUT bitor_impl,
returnvalue OUT NUMBER,
flags IN NUMBER) RETURN NUMBER
)
/
CREATE OR REPLACE TYPE BODY bitor_impl IS
STATIC FUNCTION ODCIAggregateInitialize(ctx IN OUT bitor_impl) RETURN NUMBER IS
BEGIN
ctx := bitor_impl(0);
RETURN ODCIConst.Success;
END ODCIAggregateInitialize;
MEMBER FUNCTION ODCIAggregateIterate(SELF IN OUT bitor_impl,
VALUE IN NUMBER) RETURN NUMBER IS
BEGIN
SELF.bitor := SELF.bitor + VALUE - bitand(SELF.bitor, VALUE);
RETURN ODCIConst.Success;
END ODCIAggregateIterate;
MEMBER FUNCTION ODCIAggregateMerge(SELF IN OUT bitor_impl,
ctx2 IN bitor_impl) RETURN NUMBER IS
BEGIN
SELF.bitor := SELF.bitor + ctx2.bitor - bitand(SELF.bitor, ctx2.bitor);
RETURN ODCIConst.Success;
END ODCIAggregateMerge;
MEMBER FUNCTION ODCIAggregateTerminate(SELF IN OUT bitor_impl,
returnvalue OUT NUMBER,
flags IN NUMBER) RETURN NUMBER IS
BEGIN
returnvalue := SELF.bitor;
RETURN ODCIConst.Success;
END ODCIAggregateTerminate;
END;
/
...and then define your own aggregate function
CREATE OR REPLACE FUNCTION bitoragg(x IN NUMBER) RETURN NUMBER
PARALLEL_ENABLE
AGGREGATE USING bitor_impl;
/
Usage:
SELECT user_id, bitoragg(permissions) FROM perms GROUP BY user_id

Answer 3

And you can do a bitwise or with...
FUNCTION BITOR(x IN NUMBER, y IN NUMBER)
RETURN NUMBER
AS
BEGIN
RETURN x + y - BITAND(x,y);
END;

Answer 4

You would need to know the possible permission components (1, 2 and 4) apriori (thus harder to maintain), but this is pretty simple and would work:
SELECT user_id,
MAX(BITAND(permissions, 1)) +
MAX(BITAND(permissions, 2)) +
MAX(BITAND(permissions, 4)) all_perms
FROM permissions
GROUP BY user_id

Answer 5

I'm interested to find all users who
have a particular flag set (eg: write)
on ANY record
What's wrong with simply
SELECT DISTINCT User_ID
FROM Permissions
WHERE permissions & 2 = 2