Key is not present in table - sql

Question

table description
# \d invites;
Table "public.invites"
Column | Type | Modifiers
-----------------------+-----------------------------+---------------------
id | integer | not null default
email | character varying |
key | character varying |
sender_user_id | integer | not null
receiver_user_id | integer |
Indexes:
"invites_pkey" PRIMARY KEY, btree (id)
Foreign-key constraints:
"fk_invites_receiver_user_id"
FOREIGN KEY (receiver_user_id) REFERENCES users(id)
"fk_invites_sender_user_id"
FOREIGN KEY (sender_user_id) REFERENCES users(id)
you can see Foreighn Key "fk_invites_receiver_user_id" FOREIGN KEY (receiver_user_id) REFERENCES users(id).
But the records for user with pk is absent in the parent table, where in the reference table fk is exists.
# select id from users where id = 958;
id
----
(0 rows)
select count(*) from invites where receiver_user_id = 958;
count
-------
1
(1 row)
the question is how it can be, simple way to fix the conflicts is delete wrong records, but want to exclude such situation in the future, and when i try to restore the data there is an error:
DETAIL: Key (receiver_user_id)=(958) is not present in table "users".
Command was: ALTER TABLE ONLY invites
ADD CONSTRAINT fk_invites_receiver_user_id
FOREIGN KEY (receiver_user_id) REFERENCES users(id);
P.S.
database=# select version();
version
------------------------------------------------------------------------
PostgreSQL 9.4.14 on x86_64-unknown-linux-gnu,
compiled by gcc (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4, 64-bit

Answer 1

Create fail
-- tmp schema
-- \i tmp.sql
-- tables
CREATE TABLE users
(id serial NOT NULL PRIMARY KEY
, name text
);
CREATE TABLE refs
( user_from INTEGER NOT NULL
, user_to INTEGER NOT NULL
, msg text
);
-- data
INSERT INTO users(name) VALUES ('Alice'), ('Bob');
INSERT INTO refs VALUES (1,2), (1,3);
-- FK constraints
ALTER TABLE refs
ADD CONSTRAINT bad_user_from
FOREIGN KEY (user_from) references users(id);
ALTER TABLE refs
ADD CONSTRAINT bad_user_to
FOREIGN KEY (user_to) references users(id)
NOT VALID ; -- <<--HERE
-- This should fail; user=3 does not exist
INSERT INTO refs VALUES (3,2), (2,3);
Repair
-- "repair" the broken refs (by introducing a dummy row)
INSERT INTO users(id,name) VALUES (0, 'NoName');
-- Make the bad FKs point to the dummy row
UPDATE refs r
SET user_to =0
WHERE NOT EXISTS(
SELECT *
FROM users u
WHERE u.id= r.user_to
);
UPDATE refs r
SET user_from =0
WHERE NOT EXISTS(
SELECT *
FROM users u
WHERE u.id= r.user_from
);
SELECT * FROM refs r
JOIN users u0 ON u0.id= r.user_from
JOIN users u1 ON u1.id= r.user_to
;
INSERT INTO users(name) VALUES ('NewName');
-- see what we'v got
SELECT * FROM users;
SELECT* FROM refs r
JOIN users u0 ON u0.id= r.user_from
JOIN users u1 ON u1.id= r.user_to;
Validate constraint
\d refs
\d users
-- enforce the constraint
ALTER TABLE refs
VALIDATE CONSTRAINT bad_user_to; -- <<-- HERE
-- check if valid
\d refs
\d users
-- This should not fail; user=3 does exist now
INSERT INTO refs VALUES (3,2), (2,3);

Answer 2

The answer is unfortunately “data corruption”.
You should delete the offending row, dump the database with pg_dumpall and restore it into a fresh cluster created with initdb.
You should also try to find out what caused the data corruption. Did you have any crashes recently? Could it be that you have unreliable storage or hardware problems? Anything weird in the database log?
To exclude such problems as much as possible, always use the latest fixpack for your PostgreSQL version and use reliable hardware.