Special DELETE command

Question

Hello everybody!
I don't know what does this DELETE instruction:
DELETE /*+ BYPASS_UJVC */FROM (SELECT a.ROWID
FROM TABLE1 a,
TABLE2 b,
TABLE3 c,
TABLE4 d
WHERE a.id_sipm = b.id_sipm
AND b.id_sitr_pefe = c.id_sitr_pefe
AND c.id_dece_400 = d.id_dece_400
AND d.cod_dece_tica = 'MI') e;
Delete rows ONLY from TABLE1?
Delete rows from all tables joined?
Wich is the use of this DELETE?
And the /*+ BYPASS_UJVC */ hint?
Thanks for advanced.

Answer 1

Hoek:
Interesting demonstration, but the OP needs to check a couple of things first before deciding which table will be deleted.
Same setup as yours (thanks for the scripts) but I added:
SQL> alter table table2 add constraint t2_pk primary key (col);
Table altered.
SQL> alter table table3 add constraint t3_pk primary key (col);
Table altered.
SQL> delete
  2  from (select z.rowid
  3        from table1 z, table2 a, table3 b
  4        where a.col = z.col and
  5              a.col = b.col);
5 rows deleted.
SQL> select * from table1;
no rows selected
SQL> select * from table2;
       COL
         1
         2
         3
         4
         5
SQL> select * from table3;
       COL
         1
         2
         3
         4
         5So, it deletes the non-key preserved table. I get the same result with and without the hint.
When I add a PK to table1, so now all three have PKs I get:
SQL> delete
  2  from (select z.rowid
  3        from table1 z, table2 a, table3 b
  4        where a.col = z.col and
  5              a.col = b.col);
5 rows deleted.
SQL> select * from table1;
no rows selected
SQL> select * from table2;
       COL
         1
         2
         3
         4
         5
SQL> select * from table3;
       COL
         1
         2
         3
         4
         5Again, the hint does not change the result. If I then drop the PK on table2, I get table2 deleted both with and without the hint.
So, it appears that if the query would succeed without the hint, it will delete the non-key preserved table if there is one and if not it will delete the first one mentioned with ot without the hint. If the query requires the hint to succeed, it will delete the last table mentioned, even if that is key preserved. I ran this up to 5 tables and the pattern seemed to hold. I also played around with the join orders and got the same results.
In an attempt to influence the behaviour, I attempted to use the selected rowid as a predicate to influence the table deleted. In the key preserved case table1 and table2 had PKs table3 did not. Without a predicate, table3 was deleted when I tried this delete, I was surprised to see the result.
SQL> delete
  2  from ( select z.rowid rd
  3         from table1 z, table2 a, table3 b
  4         where z.col = a.col and
  5               z.col = b.col)
  6  where rowid = rd;
0 rows deleted.I got the same result trying with a.rowid but using b.rowid in the select deleted 5 rows in table3
After dropping the PKs on table1 and table2 and trying the same thing with the hint I got:
SQL> delete /*+ bypass_ujvc */
  2  from ( select z.rowid rd
  3         from table1 z, table2 a, table3 b
  4         where z.col = a.col and
  5               z.col = b.col)
  6  where rowid = rd;
       from table1 z, table2 a, table3 b
ERROR at line 3:
ORA-01445: cannot select ROWID from, or sample, a join view without a key-preserved tableattempting to select any of the rowid values.
John
Edited by: John Spencer on Oct 30, 2010 1:32 PM
That's what I get for testing additional things while in the middle of posting, Hoek beat me to the keyed version :-(