Re: Delete a table automatic? - Mailing list pgsql-general

On 2022-11-01 07:41:14 -0600, Rob Sargent wrote:
> On 11/1/22 03:31, jian he wrote:
>
>     On Tue, Nov 1, 2022 at 2:33 PM 黄宁 <huangning0722@gmail.com> wrote:
>
>         I now have two tables named A and B. Table B is calculated based on the
>         data of table A. I wonder if table B can be automatically deleted when
>         table A is deleted？
[...]
>     you can use DROP TABLE CASCADE.
>     DROP TABLE manual: https://www.postgresql.org/docs/current/
>     sql-droptable.html
>
>
>
> Only If B has a foreign key reference to A

And even then it only drops the constraint, not the table (or the data):

hjp=> create table a (id serial primary key, t text);
CREATE TABLE
hjp=> create table b (id serial primary key, a int references a, t text);
CREATE TABLE
hjp=> \d a
                             Table "public.a"
╔════════╤═════════╤═══════════╤══════════╤═══════════════════════════════╗
║ Column │  Type   │ Collation │ Nullable │            Default            ║
╟────────┼─────────┼───────────┼──────────┼───────────────────────────────╢
║ id     │ integer │           │ not null │ nextval('a_id_seq'::regclass) ║
║ t      │ text    │           │          │                               ║
╚════════╧═════════╧═══════════╧══════════╧═══════════════════════════════╝
Indexes:
    "a_pkey" PRIMARY KEY, btree (id)
Referenced by:
    TABLE "b" CONSTRAINT "b_a_fkey" FOREIGN KEY (a) REFERENCES a(id)

hjp=> \d b
                             Table "public.b"
╔════════╤═════════╤═══════════╤══════════╤═══════════════════════════════╗
║ Column │  Type   │ Collation │ Nullable │            Default            ║
╟────────┼─────────┼───────────┼──────────┼───────────────────────────────╢
║ id     │ integer │           │ not null │ nextval('b_id_seq'::regclass) ║
║ a      │ integer │           │          │                               ║
║ t      │ text    │           │          │                               ║
╚════════╧═════════╧═══════════╧══════════╧═══════════════════════════════╝
Indexes:
    "b_pkey" PRIMARY KEY, btree (id)
Foreign-key constraints:
    "b_a_fkey" FOREIGN KEY (a) REFERENCES a(id)

[some inserts later]

hjp=> select * from b;
╔════╤═══╤══════╗
║ id │ a │  t   ║
╟────┼───┼──────╢
║  1 │ 1 │ foo1 ║
║  2 │ 1 │ foo2 ║
║  3 │ 2 │ bar1 ║
╚════╧═══╧══════╝
(3 rows)

hjp=> drop table a cascade;
NOTICE:  drop cascades to constraint b_a_fkey on table b
DROP TABLE

hjp=> \d b
                             Table "public.b"
╔════════╤═════════╤═══════════╤══════════╤═══════════════════════════════╗
║ Column │  Type   │ Collation │ Nullable │            Default            ║
╟────────┼─────────┼───────────┼──────────┼───────────────────────────────╢
║ id     │ integer │           │ not null │ nextval('b_id_seq'::regclass) ║
║ a      │ integer │           │          │                               ║
║ t      │ text    │           │          │                               ║
╚════════╧═════════╧═══════════╧══════════╧═══════════════════════════════╝
Indexes:
    "b_pkey" PRIMARY KEY, btree (id)

As you can see, the table is still there, but the foreign key constraint
is gone.

hjp=> select * from b;
╔════╤═══╤══════╗
║ id │ a │  t   ║
╟────┼───┼──────╢
║  1 │ 1 │ foo1 ║
║  2 │ 1 │ foo2 ║
║  3 │ 2 │ bar1 ║
╚════╧═══╧══════╝
(3 rows)

And the data in the table is also unchanged.

        hp

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   _  | Peter J. Holzer    | Story must make more sense than reality.
|_|_) |                    |
| |   | hjp@hjp.at         |    -- Charles Stross, "Creative writing
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