Question

How can I see which entries depend on a given entry using PostgreSQL ? (dependent meaning “having a foreign key referencing the entry”).

Basically, I want to check which entries might be cascaded when I DELETE a given entry of a table.

Answer 1

To see all actual rows depending via fk constraint, identify the columns with the tools described below.

Where foreign key constraints are defined with ON DELETE CASCADE, depending rows will be deleted (possibly cascading the DELETE to more depending tables).

Where foreign key constraints are defined with ON DELETE SET NULL / ON DELETE SET DEFAULT, only the value in the columns will be reset to NULL / default value.

Else a DELETE on rows with dependent rows would fail with an exception.

Then run queries like the following on the identified tables / columns:

SELECT f.tbl_fk_id, f.col_fk
FROM   tbl_fk f
JOIN   tbl t ON f.col_fk = t.col
AND    <same condition as DELETE here>;

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pgAdmin supplies this feature:

Pick the the object in the object browser to the left and chose the dependents pane top right.

pgAdmin uses a couple of queries to the system catalog to assemble the list. You could log the commands issued if you want to build a query yourself.

Also, when deleting an object where you are not completely sure about dependents, try a plain DROP first (without CASCADE). You will get an error message if any dependent exists …

And finally, but proceed with caution!, you can start a transaction and just issue the command:

BEGIN;
DROP TABLE tbl CASCADE;

Then, if you like what you see:

COMMIT;

If you don’t:

ROLLBACK;

And it will be like it never happened. 😉

You will see something like this:

NOTICE:  drop cascades to 4 other objects
DETAIL:  drop cascades to constraint tbl1_tbl_id_fkey on table myschema.tbl1
drop cascades to constraint tbl_winner_tbl_id_fkey on table myschema.tbl_foo
drop cascades to constraint bar_tbl_id_fkey on table myschema.bar
drop cascades to constraint tbl1_tbl_id_fkey on table x.tbl1

Query returned successfully with no result in 47 ms.