How to programmatically check if row is deletable? - sql

Question

Say we have a PostgreSQL table like so:
CREATE TABLE master (
id INT PRIMARY KEY,
...
);
and many other tables referencing it with foreign keys:
CREATE TABLE other (
id INT PRIMARY KEY,
id_master INT NOT NULL,
...
CONSTRAINT other_id_master_fkey FOREIGN KEY (id_master)
REFERENCES master (id) ON DELETE RESTRICT
);
Is there a way to check (from within trigger function) if a master row is deletable without actually trying to delete it? The obvious way is to do a SELECT on all referencing tables one by one, but I would like to know if there is an easier way.
The reason I need this is that I have a table with hierarchical data in which any row can have child rows, and only child rows that are lowest in hierarchy can be referenced by other tables. So when a row is about to become a parent row, I need to check whether it is already referenced anywhere. If it is, it cannot become a parent row, and insertion of new child row is denied.

Answer 1

You can try to delete the row and roll back the effects. You wouldn't want to do that in a trigger function because any exception cancels all persisted changes to the database. The manual:
When an error is caught by an EXCEPTION clause, the local variables of
the PL/pgSQL function remain as they were when the error occurred, but
all changes to persistent database state within the block are rolled back.
Bold emphasis mine.
But you can wrap this into a separate block or a separate plpgsql function and catch the exception there to prevent the effect on the main (trigger) function.
CREATE OR REPLACE FUNCTION f_can_del(_id int)
RETURNS boolean AS
$func$
BEGIN
DELETE FROM master WHERE master_id = _id; -- DELETE is always rolled back
IF NOT FOUND THEN
RETURN NULL; -- ID not found, return NULL
END IF;
RAISE SQLSTATE 'MYERR'; -- If DELETE, raise custom exception
EXCEPTION
WHEN FOREIGN_KEY_VIOLATION THEN
RETURN FALSE;
WHEN SQLSTATE 'MYERR' THEN
RETURN TRUE;
-- other exceptions are propagated as usual
END
$func$ LANGUAGE plpgsql;
This returns TRUE / FALSE / NULL indicating that the row can be deleted / not be deleted / does not exist.
db<>fiddle here
Old sqlfiddle
One could easily make this function dynamic to test any table / column / value.
Since PostgreSQL 9.2 you can also report back which table was blocking.
PostgreSQL 9.3 or later offer more detailed information, yet.
Generic function for arbitrary table, column and type
Why did the attempt on a dynamic function that you posted in the comments fail? This quote from the manual should give a clue:
Note in particular that EXECUTE changes the output of GET DIAGNOSTICS, but does not change FOUND.
It works with GET DIAGNOSTICS:
CREATE OR REPLACE FUNCTION f_can_del(_tbl regclass, _col text, _id int)
RETURNS boolean AS
$func$
DECLARE
_ct int; -- to receive count of deleted rows
BEGIN
EXECUTE format('DELETE FROM %s WHERE %I = $1', _tbl, _col)
USING _id; -- exception if other rows depend
GET DIAGNOSTICS _ct = ROW_COUNT;
IF _ct > 0 THEN
RAISE SQLSTATE 'MYERR'; -- If DELETE, raise custom exception
ELSE
RETURN NULL; -- ID not found, return NULL
END IF;
EXCEPTION
WHEN FOREIGN_KEY_VIOLATION THEN
RETURN FALSE;
WHEN SQLSTATE 'MYERR' THEN
RETURN TRUE;
-- other exceptions are propagated as usual
END
$func$ LANGUAGE plpgsql;
db<>fiddle here
Old sqlfiddle
While being at it, I made it completely dynamic, including the data type of the column (it has to match the given column, of course). I am using the polymorphic type anyelement for that purpose. See:
How to write a function that returns text or integer values?
I also use format() and a parameter of type regclass to safeguard against SQLi. See:
SQL injection in Postgres functions vs prepared queries

Answer 2

You can do that also with Procedure.
CREATE OR REPLACE procedure p_delable(_tbl text, _col text, _id int)
AS $$
DECLARE
_ct bigint;
_exists boolean; -- to receive count of deleted rows
BEGIN
_exists := (SELECT EXISTS ( SELECT FROM information_schema.tables
WHERE table_schema = 'public' AND table_name = $1 ));
IF _exists THEN
EXECUTE format('DELETE FROM %s WHERE %I = $1', _tbl, _col)
USING _id; -- exception if other rows depend
GET DIAGNOSTICS _ct = ROW_COUNT;
IF _ct > 0 THEN
RAISE SQLSTATE 'MYERR'; -- If DELETE, raise custom exception
ELSE
RAISE NOTICE 'no records found. no records will be deleted';
END IF;
ELSE
raise notice 'Input text is invalid table name.';
END IF;
EXCEPTION
WHEN undefined_column then
raise notice 'Input text is invalid column name.';
WHEN undefined_table then
raise notice 'Input text is invalid table name.';
WHEN FOREIGN_KEY_VIOLATION THEN
RAISE NOTICE 'foreign key violation, cannot be deleted.';
WHEN SQLSTATE 'MYERR' THEN
RAISE NOTICE 'rows % found and can be deleted.', _ct;
END
$$ LANGUAGE plpgsql;
You can call it, also can validate your input.
call p_delable('parent_tree', 'parent_id',30);
Will get:
NOTICE: no records found. no records will be deleted
Lets try an actual exist row.
call p_delable('parent_tree', 'parent_id',3);
It will return
NOTICE: rows 1 found and can be deleted.
It can also check your input table name exists in public schema or not.
call p_delable('parent_tre', 'parent_id',3);
It will give you notice:
NOTICE: Input text is invalid table name.