UPDATE FROM problem, multiple updates of same row don"t seem to

Here is an example:

CREATE TABLE tablea(
id int PRIMARY KEY,
flag int
);

CREATE TABLE tableb(
aid int REFERENCES tablea(id),
flag int
);

INSERT INTO tablea VALUES(1,0);
INSERT INTO tablea VALUES(2,0);

-- Flags for 1st row of tablea - When ORed, should be 7
INSERT INTO tableb VALUES(1,1);
INSERT INTO tableb VALUES(1,2);
INSERT INTO tableb VALUES(1,4);

-- Flags for 2nd row of tablea - When ORed, should be 5
INSERT INTO tableb VALUES(2,1);
INSERT INTO tableb VALUES(2,4);


UPDATE tablea
SET flag = tablea.flag | tableb.flag
FROM tableb
WHERE tablea.id = tableb.aid;


SELECT * from tablea;
id | flag
----+------
1 | 1
2 | 1

-- Desired output is
id | flag
----+------
1 | 7
2 | 5


Is there a way around this so that I can get the desired output?

--
David Stanaway

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Re: UPDATE FROM problem, multiple updates of same row don"t seem to work

On Mon, Aug 09, 2004 at 15:16:29 -0500,
David Stanaway wrote:
> Here is an example:
>
> CREATE TABLE tablea(
> id int PRIMARY KEY,
> flag int
> );
>
> CREATE TABLE tableb(
> aid int REFERENCES tablea(id),
> flag int
> );
>
> INSERT INTO tablea VALUES(1,0);
> INSERT INTO tablea VALUES(2,0);
>
> -- Flags for 1st row of tablea - When ORed, should be 7
> INSERT INTO tableb VALUES(1,1);
> INSERT INTO tableb VALUES(1,2);
> INSERT INTO tableb VALUES(1,4);
>
> -- Flags for 2nd row of tablea - When ORed, should be 5
> INSERT INTO tableb VALUES(2,1);
> INSERT INTO tableb VALUES(2,4);
>
>
> UPDATE tablea
> SET flag = tablea.flag | tableb.flag

The original value of tablea.flag for each id will be used here. So that
only one of the tableb.flag values will be or'd in for each id.

> FROM tableb
> WHERE tablea.id = tableb.aid;
>
>
> SELECT * from tablea;
> id | flag
> ----+------
> 1 | 1
> 2 | 1
>
> -- Desired output is
> id | flag
> ----+------
> 1 | 7
> 2 | 5
>
>
> Is there a way around this so that I can get the desired output?

Write a custom aggregate function that does the or for you.

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Re: UPDATE FROM problem, multiple updates of same row don"t seem to work

On Wed, Aug 11, 2004 at 20:50:28 -0500,
David Stanaway wrote:
>
> I had thought about that, but this is a simpler case of what I need to
> do. The operations for each column in the update are dependent on the
> current and new values of each row being merged.
>
> Currently I am doing this with a cursor which is very slow! How about a
> trigger on update? Or would this suffer the same behavior that I am
> seeing with UPDATE FROM with a 1-many join?

There is a good chance that triggers would be faster since tablea seems
to reallly be a materialized view of an aggregate over tableb and doing
a delta calculation when a row changes is going to be faster than recalculating
the aggregate from scratch.

You won't have the same problem if you use a trigger as when you were
trying to calculate an aggregate using UPDATE.

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