Thread: [GENERAL] Making a unique constraint deferrable?
Hello,
If I'm interpreting the manual correctly, this should work:
ivoras=# create table foo(a integer, b integer, unique(a,b));
CREATE TABLE
ivoras=# \d foo
Table "public.foo"
Column | Type | Modifiers
--------+---------+-----------
a | integer |
b | integer |
Indexes:
"foo_a_b_key" UNIQUE CONSTRAINT, btree (a, b)
ivoras=# insert into foo(a,b) values(1,2);
INSERT 0 1
ivoras=# insert into foo(a,b) values(1,2);
ERROR: duplicate key value violates unique constraint "foo_a_b_key"
DETAIL: Key (a, b)=(1, 2) already exists.
ivoras=# alter table foo alter constraint "foo_a_b_key" deferrable;
ERROR: constraint "foo_a_b_key" of relation "foo" is not a foreign key constraint
The manual says this for SET CONSTRAINTS:
Currently, only UNIQUE, PRIMARY KEY, REFERENCES (foreign key), and EXCLUDE constraints are affected by this setting. NOT NULL and CHECK constraints are always checked immediately when a row is inserted or modified (not at the end of the statement). Uniqueness and exclusion constraints that have not been declared DEFERRABLE are also checked immediately.
I'm puzzled by the "...is not a foreign key constraint" error message. Doesn't "deferrable" also work on unique constraints?
On 02/28/2017 08:50 AM, Ivan Voras wrote: > Hello, > > If I'm interpreting the manual correctly, this should work: > > ivoras=# create table foo(a integer, b integer, unique(a,b)); > CREATE TABLE > ivoras=# \d foo > Table "public.foo" > Column | Type | Modifiers > --------+---------+----------- > a | integer | > b | integer | > Indexes: > "foo_a_b_key" UNIQUE CONSTRAINT, btree (a, b) > > ivoras=# insert into foo(a,b) values(1,2); > INSERT 0 1 > ivoras=# insert into foo(a,b) values(1,2); > ERROR: duplicate key value violates unique constraint "foo_a_b_key" > DETAIL: Key (a, b)=(1, 2) already exists. > ivoras=# alter table foo alter constraint "foo_a_b_key" deferrable; > ERROR: constraint "foo_a_b_key" of relation "foo" is not a foreign key > constraint > > The manual says this for SET CONSTRAINTS: > > Currently, only UNIQUE, PRIMARY KEY, REFERENCES (foreign key), and > EXCLUDE constraints are affected by this setting. NOT NULL and CHECK > constraints are always checked immediately when a row is inserted or > modified (not at the end of the statement). Uniqueness and exclusion > constraints that have not been declared DEFERRABLE are also checked > immediately. > > > I'm puzzled by the "...is not a foreign key constraint" error message. > Doesn't "deferrable" also work on unique constraints? https://www.postgresql.org/docs/9.6/static/sql-altertable.html "ALTER CONSTRAINT This form alters the attributes of a constraint that was previously created. Currently only foreign key constraints may be altered. " > > -- Adrian Klaver adrian.klaver@aklaver.com
Hello,If I'm interpreting the manual correctly, this should work:ivoras=# create table foo(a integer, b integer, unique(a,b));CREATE TABLEivoras=# \d fooTable "public.foo"Column | Type | Modifiers--------+---------+-----------a | integer |b | integer |Indexes:"foo_a_b_key" UNIQUE CONSTRAINT, btree (a, b)ivoras=# insert into foo(a,b) values(1,2);INSERT 0 1ivoras=# insert into foo(a,b) values(1,2);ERROR: duplicate key value violates unique constraint "foo_a_b_key"DETAIL: Key (a, b)=(1, 2) already exists.ivoras=# alter table foo alter constraint "foo_a_b_key" deferrable;ERROR: constraint "foo_a_b_key" of relation "foo" is not a foreign key constraintThe manual says this for SET CONSTRAINTS:Currently, only UNIQUE, PRIMARY KEY, REFERENCES (foreign key), and EXCLUDE constraints are affected by this setting. NOT NULL and CHECK constraints are always checked immediately when a row is inserted or modified (not at the end of the statement). Uniqueness and exclusion constraints that have not been declared DEFERRABLE are also checked immediately.I'm puzzled by the "...is not a foreign key constraint" error message. Doesn't "deferrable" also work on unique constraints?
The error is pointing out the documented behavior that only FK constraints can be altered.
https://www.postgresql.org/docs/9.6/static/sql-altertable.html (see ALTER CONSTRAINT note)
So, while you can make a PK constraint deferrable it must be done as part of the initial constraint construction and not via ALTER CONSTRAINT.
David J.
Hello,If I'm interpreting the manual correctly, this should work:ivoras=# create table foo(a integer, b integer, unique(a,b));CREATE TABLEivoras=# \d fooTable "public.foo"Column | Type | Modifiers--------+---------+-----------a | integer |b | integer |Indexes:"foo_a_b_key" UNIQUE CONSTRAINT, btree (a, b)ivoras=# insert into foo(a,b) values(1,2);INSERT 0 1ivoras=# insert into foo(a,b) values(1,2);ERROR: duplicate key value violates unique constraint "foo_a_b_key"DETAIL: Key (a, b)=(1, 2) already exists.ivoras=# alter table foo alter constraint "foo_a_b_key" deferrable;ERROR: constraint "foo_a_b_key" of relation "foo" is not a foreign key constraintThe manual says this for SET CONSTRAINTS:Currently, only UNIQUE, PRIMARY KEY, REFERENCES (foreign key), and EXCLUDE constraints are affected by this setting. NOT NULL and CHECK constraints are always checked immediately when a row is inserted or modified (not at the end of the statement). Uniqueness and exclusion constraints that have not been declared DEFERRABLE are also checked immediately.I'm puzzled by the "...is not a foreign key constraint" error message. Doesn't "deferrable" also work on unique constraints?
The error is pointing out the documented behavior that only FK constraints can be altered.
https://www.postgresql.org/docs/9.6/static/sql-altertable.html (see ALTER CONSTRAINT note)
So, while you can make a PK constraint deferrable it must be done as part of the initial constraint construction and not via ALTER CONSTRAINT.
David J.
ivoras=# alter table foo alter constraint "foo_a_b_key" deferrable;ERROR: constraint "foo_a_b_key" of relation "foo" is not a foreign key constraint
A more clear error message would be:
EROR: cannot alter non-foreign key constraint "foo_a_b_key" of relation "foo"
Though I'm not sure how that meshes with the error message style guides...
David J.
On 28 February 2017 at 18:03, David G. Johnston <david.g.johnston@gmail.com> wrote:
ivoras=# alter table foo alter constraint "foo_a_b_key" deferrable;ERROR: constraint "foo_a_b_key" of relation "foo" is not a foreign key constraintA more clear error message would be:EROR: cannot alter non-foreign key constraint "foo_a_b_key" of relation "foo"Though I'm not sure how that meshes with the error message style guides...
Any idea what underlying technical reason prohibits marking non-fk constraints as deferrable?
On 28 February 2017 at 18:03, David G. Johnston <david.g.johnston@gmail.com> wrote:ivoras=# alter table foo alter constraint "foo_a_b_key" deferrable;ERROR: constraint "foo_a_b_key" of relation "foo" is not a foreign key constraintA more clear error message would be:EROR: cannot alter non-foreign key constraint "foo_a_b_key" of relation "foo"Though I'm not sure how that meshes with the error message style guides...Any idea what underlying technical reason prohibits marking non-fk constraints as deferrable?
Not off hand - but a unique (and PK by extension) constraint is implemented by creating an underlying unique index and the ALTER CONSTRAINT command probably doesn't want to go messing around with that. While the columns involved in a FK constraint can also be indexed the two concepts are not physically linked together.
David J.