ORACLE : WITH CLAUSE & FOR UPDATE NOWAIT - sql

Question

In a simple join, I would like to limit the results of the first table. So I thought about doing this :
WITH events AS (SELECT event FROM risk_event WHERE status = 'ABC' AND rownum <= 20)
SELECT event_id
FROM events ev, attributes att
WHERE ev.event_id = att.risk_event_id
FOR UPDATE NOWAIT
The problem is that I get an ORA-02014: cannot select FOR UPDATE from view exception because of the rownum<=20 and the FOR UPDATE NOWAIT'.
I know that I can do it with a inner in clause as well, but I'm wondering if there is a better way?

Answer 1

Try first select rowid and then query with table from which you select this rowid
DDL:
create table risk_event as select level as event, mod(level,20) as status from dual connect by level <=10000;
begin
dbms_stats.gather_table_stats(user,
'risk_event',
cascade => true,
estimate_percent => null,
method_opt => 'for all columns size 1');
end;
/
create table attributes as select * from risk_event;
begin
dbms_stats.gather_table_stats(user,
'attributes',
cascade => true,
estimate_percent => null,
method_opt => 'for all columns size 1');
end;
/
Code
WITH events AS (SELECT rowid as rd from risk_event WHERE status = 19 AND rownum <= 20)
SELECT ev.*
FROM risk_event ev, attributes att
WHERE ev.event = att.event and ev.rowid in(select rd from events)
FOR UPDATE NOWAIT
Plan
-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost | Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 23 | 11 | 00:00:01 |
| 1 | FOR UPDATE | | | | | |
| 2 | BUFFER SORT | | | | | |
| * 3 | HASH JOIN | | 1 | 23 | 11 | 00:00:01 |
| 4 | NESTED LOOPS | | 1 | 19 | 4 | 00:00:01 |
| 5 | VIEW | VW_NSO_1 | 20 | 240 | 2 | 00:00:01 |
| 6 | SORT UNIQUE | | 1 | 240 | | |
| 7 | VIEW | | 20 | 240 | 2 | 00:00:01 |
| * 8 | COUNT STOPKEY | | | | | |
| * 9 | TABLE ACCESS FULL | RISK_EVENT | 20 | 140 | 2 | 00:00:01 |
| 10 | TABLE ACCESS BY USER ROWID | RISK_EVENT | 1 | 7 | 1 | 00:00:01 |
| 11 | TABLE ACCESS FULL | ATTRIBUTES | 10000 | 40000 | 7 | 00:00:01 |
-----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
------------------------------------------
* 3 - access("EV"."EVENT"="ATT"."EVENT")
* 8 - filter(ROWNUM<=20)
* 9 - filter("STATUS"=19)