CHECK Constraint Deferrable

Hi,

Currently, there is no support for CHECK constraint DEFERRABLE in a create table statement.

SQL standard specifies that CHECK constraint can be defined as DEFERRABLE.

The attached patch is having implementation for CHECK constraint Deferrable as below:

‘postgres[579453]=#’CREATE TABLE t1 (i int CHECK(i<>0) DEFERRABLE, t text);
CREATE TABLE
‘postgres[579453]=#’\d t1
                 Table "public.t1"
 Column |  Type   | Collation | Nullable | Default
--------+---------+-----------+----------+---------
 i      | integer |           |          |
 t      | text    |           |          |
Check constraints:
    "t1_i_check" CHECK (i <> 0) DEFERRABLE

Now we can have a deferrable CHECK constraint, and we can defer the constraint validation:

‘postgres[579453]=#’BEGIN;
BEGIN
‘postgres[579453]=#*’SET CONSTRAINTS t1_i_check DEFERRED;
SET CONSTRAINTS
‘postgres[579453]=#*’INSERT INTO t1 VALUES (0, 'one'); -- should succeed
INSERT 0 1
‘postgres[579453]=#*’UPDATE t1 SET i = 1 WHERE t = 'one';
UPDATE 1
‘postgres[579453]=#*’COMMIT; -- should succeed
COMMIT

Attaching the initial patch, I will improve it with documentation in my next version of the patch.

thoughts?