Error "duplicate key value violates unique constraint" while updating multiple rows - sql

Question

I created a table in PostgreSQL and Oracle as
CREATE TABLE temp(
seqnr smallint NOT NULL,
defn_id int not null,
attr_id int not null,
input CHAR(50) NOT NULL,
CONSTRAINT pk_id PRIMARY KEY (defn_id, attr_id, seqnr)
);
This temp table has primary key as (defn_id,attr_id,seqnr) as a whole!
Then I inserted the record in the temp table as
INSERT INTO temp(seqnr,defn_id,attr_id,input)
VALUES (1,100,100,'test1');
INSERT INTO temp(seqnr,defn_id,attr_id,input)
VALUES (2,100,100,'test2');
INSERT INTO temp(seqnr,defn_id,attr_id,input)
VALUES (3,100,100,'test3');
INSERT INTO temp(seqnr,defn_id,attr_id,input)
VALUES (4,100,100,'test4');
INSERT INTO temp(seqnr,defn_id,attr_id,input)
VALUES (5,100,100,'test5');
in both Oracle and Postgres!
The table now contains:
seqnr | defn_id | attr_id | input
1 | 100 | 100 | test1
2 | 100 | 100 | test2
3 | 100 | 100 | test3
4 | 100 | 100 | test4
5 | 100 | 100 | test5
When I run the command:
UPDATE temp SET seqnr=seqnr+1
WHERE defn_id = 100 AND attr_id = 100 AND seqnr >= 1;
In case of ORACLE it is Updating 5 Rows and the O/p is
seqnr | defn_id | attr_id | input
2 | 100 | 100 | test1
3 | 100 | 100 | test2
4 | 100 | 100 | test3
5 | 100 | 100 | test4
6 | 100 | 100 | test5
But in case of PostgreSQL it is giving an error!
DETAIL: Key (defn_id, attr_id, seqnr)=(100, 100, 2) already exists.
Why does this happen and how can I replicate the same result in Postgres as Oracle?
Or how can the same result be achieved in Postgres without any errors?

Answer 1

UNIQUE an PRIMARY KEY constraints are checked immediately (for each row) unless they are defined DEFERRABLE - which is the solution you demand.
ALTER TABLE temp
DROP CONSTRAINT pk_id
, ADD CONSTRAINT pk_id PRIMARY KEY (defn_id, attr_id, seqnr) DEFERRABLE
;
Then your UPDATE just works.
db<>fiddle here
This comes at a cost, though. The manual:
Note that deferrable constraints cannot be used as conflict
arbitrators in an INSERT statement that includes an ON CONFLICT DO UPDATE clause.
And for FOREIGN KEY constraints:
The referenced columns must be the columns of a non-deferrable unique
or primary key constraint in the referenced table.
And:
When a UNIQUE or PRIMARY KEY constraint is not deferrable,
PostgreSQL checks for uniqueness immediately whenever a row is
inserted or modified. The SQL standard says that uniqueness should be
enforced only at the end of the statement; this makes a difference
when, for example, a single command updates multiple key values. To
obtain standard-compliant behavior, declare the constraint as
DEFERRABLE but not deferred (i.e., INITIALLY IMMEDIATE). Be aware
that this can be significantly slower than immediate uniqueness
checking.
See:
Constraint defined DEFERRABLE INITIALLY IMMEDIATE is still DEFERRED?
I would avoid a DEFERRABLE PK if at all possible. Maybe you can work around the demonstrated problem? This usually works:
UPDATE temp t
SET seqnr = t.seqnr + 1
FROM (
SELECT defn_id, attr_id, seqnr
FROM temp
WHERE defn_id = 100 AND attr_id = 100 AND seqnr >= 1
ORDER BY defn_id, attr_id, seqnr DESC
) o
WHERE (t.defn_id, t.attr_id, t.seqnr)
= (o.defn_id, o.attr_id, o.seqnr);
db<>fiddle here
But there are no guarantees as ORDER BY is not specified for UPDATE in Postgres.
Related:
UPDATE with ORDER BY