Dropping dependent tables

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Dropping dependent tables

Tiffany Thang
Hi,
I would like to write a SQL statement to drop all the tables owned by me but a problem I’m struggling with is with referential integrity. The statement I have now to generate the drop statements is

select 'drop table '||tablename||' cascade;' from pg_tables where tableowner='<myuseraccount>';

The generated SQLs above might attempt to drop the parent tables first before the child and to be able to drop all the tables, I had to run the SQL script in multiple iterations. Not very clean.

Can someone advise how I could formulate the SQL to check for table dependencies to generate a SQL script that drops the child tables first before the parent? Or are there any better alternatives?

Thank you.

Tiff

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Re: Dropping dependent tables

Tom Lane-2
Tiffany Thang <[hidden email]> writes:
> I would like to write a SQL statement to drop all the tables owned by me
> but a problem I’m struggling with is with referential integrity. The
> statement I have now to generate the drop statements is

> select 'drop table '||tablename||' cascade;' from pg_tables where
> tableowner='<myuseraccount>';

> The generated SQLs above might attempt to drop the parent tables first
> before the child and to be able to drop all the tables, I had to run the
> SQL script in multiple iterations. Not very clean.

Uh ... it's not clear to me why that wouldn't work.  CASCADE should
be able to take care of foreign keys:

postgres=# create table t1 (f1 int primary key);
CREATE TABLE
postgres=# create table t2 (f2 int references t1);
CREATE TABLE
postgres=# drop table t1;
ERROR:  cannot drop table t1 because other objects depend on it
DETAIL:  constraint t2_f2_fkey on table t2 depends on table t1
HINT:  Use DROP ... CASCADE to drop the dependent objects too.
postgres=# drop table t1 cascade;
NOTICE:  drop cascades to constraint t2_f2_fkey on table t2
DROP TABLE

Could you enlarge on what problem you saw, specifically?

                        regards, tom lane


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Re: Dropping dependent tables

Tiffany Thang

On Thu, Jun 3, 2021 at 10:18 AM Tom Lane <[hidden email]> wrote:
Tiffany Thang <[hidden email]> writes:
> I would like to write a SQL statement to drop all the tables owned by me
> but a problem I’m struggling with is with referential integrity. The
> statement I have now to generate the drop statements is

> select 'drop table '||tablename||' cascade;' from pg_tables where
> tableowner='<myuseraccount>';

> The generated SQLs above might attempt to drop the parent tables first
> before the child and to be able to drop all the tables, I had to run the
> SQL script in multiple iterations. Not very clean.

Uh ... it's not clear to me why that wouldn't work.  CASCADE should
be able to take care of foreign keys:

postgres=# create table t1 (f1 int primary key);
CREATE TABLE
postgres=# create table t2 (f2 int references t1);
CREATE TABLE
postgres=# drop table t1;
ERROR:  cannot drop table t1 because other objects depend on it
DETAIL:  constraint t2_f2_fkey on table t2 depends on table t1
HINT:  Use DROP ... CASCADE to drop the dependent objects too.
postgres=# drop table t1 cascade;
NOTICE:  drop cascades to constraint t2_f2_fkey on table t2
DROP TABLE

Could you enlarge on what problem you saw, specifically?

                        regards, tom lane

Thanks Tom for your response. I tried it again and I was not able to reproduce the issue.

Obviously I did something incorrectly previously.

Thanks.

Tiff