Update statement takes too long to run

Question

Hello,
I am running this simple update statement, but it takes too long to run. It was running for 16 hours and then I cancelled it. It was not even finished. The destination table that I am updating has 2.6 million records, but I am only updating 206K records. If add ROWNUM <20 to the update statement works just fine and updates the right column with the right information. Do you have any ideas what could be wrong in my update statement? I am also using a DB link since CAP.ESS_LOOKUP table resides in different db from the destination table. We are running 11g Oracle Db.
UPDATE DEV_OCS.DOCMETA IPM
SET IPM.XIPM_APP_2_17 = (SELECT DISTINCT LKP.DOC_STATUS
FROM [email protected] LKP
WHERE LKP.DOC_NUM = IPM.XIPM_APP_2_1 AND
IPM.XIPMSYS_APP_ID = 2
WHERE
IPM.XIPMSYS_APP_ID = 2;
Thanks,
Ilya

Answer 1

matthew_morris wrote:
In the first SQL, the SELECT against the remote table was a correlated subquery. the 'WHERE LKP.DOC_NUM = IPM.XIPM_APP_2_1 AND IPM.XIPMSYS_APP_ID = 2" means that the subquery had to run once for each row of DEV_OCS.DOCMETA being evaluated. This might have meant thousands of iterations, meaning a great deal of network traffic (not to mention each performing a DISTINCT operation). Queries where the data is split between two or more databases are much more expensive than queries using only tables in a single database.Sorry to disappoint you again, but with clause by itself doesn't prevent from "subquery had to run once for each row of DEV_OCS.DOCMETA being evaluated". For example:
{code}
SQL> set linesize 132
SQL> explain plan for
2 update emp e
3 set deptno = (select t.deptno from dept@sol10 t where e.deptno = t.deptno)
4 /
Explained.
SQL> @?\rdbms\admin\utlxpls
PLAN_TABLE_OUTPUT
Plan hash value: 3247731149
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|
| 0 | UPDATE STATEMENT | | 14 | 42 | 17 (83)| 00:00:01 | | |
| 1 | UPDATE | EMP | | | | | | |
| 2 | TABLE ACCESS FULL| EMP | 14 | 42 | 3 (0)| 00:00:01 | | |
| 3 | REMOTE | DEPT | 1 | 13 | 0 (0)| 00:00:01 | SOL10 | R->S |
PLAN_TABLE_OUTPUT
Remote SQL Information (identified by operation id):
3 - SELECT "DEPTNO" FROM "DEPT" "T" WHERE "DEPTNO"=:1 (accessing 'SOL10' )
16 rows selected.
SQL> explain plan for
2 update emp e
3 set deptno = (with t as (select * from dept@sol10) select t.deptno from t where e.deptno = t.deptno)
4 /
Explained.
SQL> @?\rdbms\admin\utlxpls
PLAN_TABLE_OUTPUT
Plan hash value: 3247731149
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|
| 0 | UPDATE STATEMENT | | 14 | 42 | 17 (83)| 00:00:01 | | |
| 1 | UPDATE | EMP | | | | | | |
| 2 | TABLE ACCESS FULL| EMP | 14 | 42 | 3 (0)| 00:00:01 | | |
| 3 | REMOTE | DEPT | 1 | 13 | 0 (0)| 00:00:01 | SOL10 | R->S |
PLAN_TABLE_OUTPUT
Remote SQL Information (identified by operation id):
3 - SELECT "DEPTNO" FROM "DEPT" "DEPT" WHERE "DEPTNO"=:1 (accessing 'SOL10' )
16 rows selected.
SQL>
{code}
As you can see, WITH clause by itself guaranties nothing. We must force optimizer to materialize it:
{code}
SQL> explain plan for
2 update emp e
3 set deptno = (with t as (select /*+ materialize */ * from dept@sol10) select t.deptno from t where e.deptno = t.deptno
4 /
Explained.
SQL> @?\rdbms\admin\utlxpls
PLAN_TABLE_OUTPUT
Plan hash value: 3568118945
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT|
| 0 | UPDATE STATEMENT | | 14 | 42 | 87 (17)| 00:00:02 | | |
| 1 | UPDATE | EMP | | | | | | |
| 2 | TABLE ACCESS FULL | EMP | 14 | 42 | 3 (0)| 00:00:01 | | |
| 3 | TEMP TABLE TRANSFORMATION | | | | | | | |
| 4 | LOAD AS SELECT | SYS_TEMP_0FD9D6603_1CEEEBC | | | | | | |
| 5 | REMOTE | DEPT | 4 | 80 | 3 (0)| 00:00:01 | SOL10 | R->S |
PLAN_TABLE_OUTPUT
|* 6 | VIEW | | 4 | 52 | 2 (0)| 00:00:01 | | |
| 7 | TABLE ACCESS FULL | SYS_TEMP_0FD9D6603_1CEEEBC | 4 | 80 | 2 (0)| 00:00:01 | | |
Predicate Information (identified by operation id):
6 - filter("T"."DEPTNO"=:B1)
Remote SQL Information (identified by operation id):
PLAN_TABLE_OUTPUT
5 - SELECT "DEPTNO","DNAME","LOC" FROM "DEPT" "DEPT" (accessing 'SOL10' )
25 rows selected.
SQL>
{code}
I do know hint materialize is not documented, but I don't know any other way besides splitting statement in two to materialize it.
SY.