UPDATE on seemingly key preserving view in Oracle raises ORA-01779 - sql

Question

Problem
I'm trying to refactor a low-performing MERGE statement to an UPDATE statement in Oracle 12.1.0.2.0. The MERGE statement looks like this:
MERGE INTO t
USING (
SELECT t.rowid rid, u.account_no_new
FROM t, u, v
WHERE t.account_no = u.account_no_old
AND t.contract_id = v.contract_id
AND v.tenant_id = u.tenant_id
) s
ON (t.rowid = s.rid)
WHEN MATCHED THEN UPDATE SET t.account_no = s.account_no_new
It is mostly low performing because there are two expensive accesses to the large (100M rows) table t
Schema
These are the simplified tables involved:
t The target table whose account_no column is being migrated.
u The migration instruction table containing a account_no_old → account_no_new mapping
v An auxiliary table modelling a to-one relationship between contract_id and tenant_id
The schema is:
CREATE TABLE v (
contract_id NUMBER(18) NOT NULL PRIMARY KEY,
tenant_id NUMBER(18) NOT NULL
);
CREATE TABLE t (
t_id NUMBER(18) NOT NULL PRIMARY KEY,
-- tenant_id column is missing here
account_no NUMBER(18) NOT NULL,
contract_id NUMBER(18) NOT NULL REFERENCES v
);
CREATE TABLE u (
u_id NUMBER(18) NOT NULL PRIMARY KEY,
tenant_id NUMBER(18) NOT NULL,
account_no_old NUMBER(18) NOT NULL,
account_no_new NUMBER(18) NOT NULL,
UNIQUE (tenant_id, account_no_old)
);
I cannot modify the schema. I'm aware that adding t.tenant_id would solve the problem by preventing the JOIN to v
Alternative MERGE doesn't work:
ORA-38104: Columns referenced in the ON Clause cannot be updated
Note, the self join cannot be avoided, because this alternative, equivalent query leads to ORA-38104:
MERGE INTO t
USING (
SELECT u.account_no_old, u.account_no_new, v.contract_id
FROM u, v
WHERE v.tenant_id = u.tenant_id
) s
ON (t.account_no = s.account_no_old AND t.contract_id = s.contract_id)
WHEN MATCHED THEN UPDATE SET t.account_no = s.account_no_new
UPDATE view doesn't work:
ORA-01779: cannot modify a column which maps to a non-key-preserved table
Intuitively, I would apply transitive closure here, which should guarantee that for each updated row in t, there can be only at most 1 row in u and in v. But apparently, Oracle doesn't recognise this, so the following UPDATE statement doesn't work:
UPDATE (
SELECT t.account_no, u.account_no_new
FROM t, u, v
WHERE t.account_no = u.account_no_old
AND t.contract_id = v.contract_id
AND v.tenant_id = u.tenant_id
)
SET account_no = account_no_new
The above raises ORA-01779. Adding the undocumented hint /*+BYPASS_UJVC*/ does not seem to work anymore on 12c.
How to tell Oracle that the view is key preserving?
In my opinion, the view is still key preserving, i.e. for each row in t, there is exactly one row in v, and thus at most one row in u. The view should thus be updatable. Is there any way to rewrite this query to make Oracle trust my judgement?
Or is there any other syntax I'm overlooking that prevents the MERGE statement's double access to t?

Answer 1

Is there any way to rewrite this query to make Oracle trust my judgement?
I've managed to "convince" Oracle to do MERGE by introducing helper column in target:
MERGE INTO (SELECT (SELECT t.account_no FROM dual) AS account_no_temp,
t.account_no, t.contract_id
FROM t) t
USING (
SELECT u.account_no_old, u.account_no_new, v.contract_id
FROM u, v
WHERE v.tenant_id = u.tenant_id
) s
ON (t.account_no_temp = s.account_no_old AND t.contract_id = s.contract_id)
WHEN MATCHED THEN UPDATE SET t.account_no = s.account_no_new;
db<>fiddle demo
EDIT
A variation of idea above - subquery moved directly to ON part:
MERGE INTO (SELECT t.account_no, t.contract_id FROM t) t
USING (
SELECT u.account_no_old, u.account_no_new, v.contract_id
FROM u, v
WHERE v.tenant_id = u.tenant_id
) s
ON ((SELECT t.account_no FROM dual) = s.account_no_old
AND t.contract_id = s.contract_id)
WHEN MATCHED THEN UPDATE SET t.account_no = s.account_no_new;
db<>fiddle demo2
Related article: Columns referenced in the ON Clause cannot be updated
EDIT 2:
MERGE INTO (SELECT t.account_no, t.contract_id FROM t) t
USING (SELECT u.account_no_old, u.account_no_new, v.contract_id
FROM u, v
WHERE v.tenant_id = u.tenant_id) s
ON((t.account_no,t.contract_id,'x')=((s.account_no_old,s.contract_id,'x')) OR 1=2)
WHEN MATCHED THEN UPDATE SET t.account_no = s.account_no_new;
db<>fiddle demo3

Answer 2

You may define a temporary table containing the pre-joined data from U and V.
Back it with a unique index on contract_id, account_no_old (which should be unique).
Then you may use this temporary table in an updateable join view.
create table tmp as
SELECT v.contract_id, u.account_no_old, u.account_no_new
FROM u, v
WHERE v.tenant_id = u.tenant_id;
create unique index tmp_ux1 on tmp ( contract_id, account_no_old);
UPDATE (
SELECT t.account_no, tmp.account_no_new
FROM t, tmp
WHERE t.account_no = tmp.account_no_old
AND t.contract_id = tmp.contract_id
)
SET account_no = account_no_new
;

Answer 3

Trying to do this with a simpler update. Still requires a subselect.
update t
set t.account_no = (SELECT u.account_no_new
FROM u, v
WHERE t.account_no = u.account_no_old
AND t.contract_id = v.contract_id
AND v.tenant_id = u.tenant_id);
Bobby