----- Original Message -----
From: Dan Kennedy <danielk1...@gmail.com>
To: sqlite-users@mailinglists.sqlite.org <sqlite-users@mailinglists.sqlite.org>
Sent: Friday, November 23, 2018, 16:30:12
Subject: [sqlite] Bug?: unexpected behaviour - alter parent table in a deferred
foreign key relationship
On 11/23/2018 09:54 PM, szmate1618 wrote:
Dear list members,
I have the following problem, with which I'd like to request your aid:
Currently, at version 3.25.2, SQLite only has a limited support for alter
table. E.g. you cannot change the datatype (type affinity) of a column, or
drop a column.
The usual workaround is to create a new table with the desired schema, fill
it with data from the original table, drop the original table, and rename
the new one. But what if the original table is a parent table in a foreign
key relationship?
The official solution
<https://www.sqlite.org/lang_altertable.html#otheralter> is turning foreign
keys off, making the changes you want, then turning foreign keys on. But
I'm slightly annoyed this cannot be done in a transaction (because these PRAGMA
foreign_keys =s don't take effect inside of transactions, so they need to
be issued before and after).
"PRAGMA foreign_keys = ?" is a property of the connection only, not the
database file. So what advantage would there be in including the PRAGMA
statements in the body of a transaction?
Dan.
I'd like to use deferred foreign keys instead. I have 3 queries, one of
them seems to work, the two others do not. My questions are the following:
- Does the seemingly working query work by design? Or it's just a
fortunate(?) constellation of multiple factors, and depending on other
tables or new data in the database it might break in the future? Somewhat
like undefined behavior in C++?
- Why do the other ones not work? How are they different from the first
one?
Setup
PRAGMA foreign_keys = OFF;
DROP TABLE IF EXISTS Parent;CREATE TABLE Parent(A TEXT UNIQUE,
COLUMN_TO_DROP FLOAT);INSERT INTO Parent VALUES('whatever', 0.0);
DROP TABLE IF EXISTS Child;CREATE TABLE Child(C TEXT REFERENCES
Parent(A) DEFERRABLE INITIALLY DEFERRED);INSERT INTO Child
VALUES('whatever');
PRAGMA foreign_keys = ON;
Query1 - seems to be working as intended
BEGIN TRANSACTION;CREATE TABLE Temp AS SELECT A FROM Parent;DROP TABLE
Parent;CREATE TABLE Parent (A TEXT UNIQUE);INSERT INTO Parent SELECT *
FROM Temp;DROP TABLE Temp;COMMIT;
Query2 - create [...] as select [...] fails
BEGIN TRANSACTION;CREATE TABLE Temp AS SELECT A FROM Parent;DROP TABLE
Parent;CREATE TABLE Parent AS SELECT * FROM Temp; -- different
from Query1CREATE UNIQUE INDEX ParentIndex on Parent(A); -- different
from Query1DROP TABLE Temp;COMMIT;
Result:
sqlite> PRAGMA foreign_key_check;
sqlite> .schemaCREATE TABLE Child(C TEXT REFERENCES Parent(A)
DEFERRABLE INITIALLY DEFERRED);CREATE TABLE Parent(A TEXT);CREATE
UNIQUE INDEX ParentIndex on Parent(A);
sqlite> SELECT * FROM Parent;
whatever
Query3 - insert into [...] fails
BEGIN TRANSACTION;CREATE TABLE Temp (A TEXT UNIQUE); -- different
from Query1INSERT INTO Temp SELECT A FROM Parent; -- different from
Query1DROP TABLE Parent;CREATE TABLE Parent (A TEXT UNIQUE);INSERT
INTO Parent SELECT * FROM Temp;DROP TABLE Temp;COMMIT;
Result:
sqlite> PRAGMA foreign_key_check;
sqlite> .schemaCREATE TABLE Child(C TEXT REFERENCES Parent(A)
DEFERRABLE INITIALLY DEFERRED);CREATE TABLE Parent (A TEXT UNIQUE);
sqlite> SELECT * FROM Parent;
whatever
Note that PRAGMA foreign_key_check does not report any problem in any of
the cases.
I posted an identical question on StackOverflow, but no one was able to
provide any information so far. Thanks in advance!
Máté Szabó
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