Mohamed Houri’s Oracle Notes

February 24, 2015

Parallel Query, Broadcast distribution and TEMP space

Filed under: Parallel Processing — hourim @ 6:24 pm

When it comes to parallel operations there are fundamentally two data distribution methods between individual parallel (PX) server sets: hash-hash and broadcast distribution. Suppose that you are going to operate a parallel HASH JOIN between two full table scans using a Degree of Parallelism (DOP) of 2. Since the HASH JOIN operation needs two distinct set of PX servers there will be actually always 2*DOP = 4 slaves acting in parallel. They represent two slaves per PX set: (PX1 and PX2) for the first set and (PX3 and PX4) for the second set. Each table in the join is read by one of the PX server set in parallel. When both PX servers have finished collecting their data set they need to distribute it to the subsequent parallel HASH JOIN operation. This data distribution can be done using, commonly, either a hash-hash or a broadcast distribution. Using the former method (hash-hash) the result set gathered by the parallel scan of the build and the probe tablesin the join are both sent to the parallel server set responsible for the hash join operation. Using a BROADCAST distribution method, instead of distributing rows from both result sets Oracle sends the smaller result set to all parallel server slaves of the set responsible of the subsequent parallel hash joins operation.

I bolded the word smaller result set to emphasize that smaller is relative to the other row source in the join. Do you consider that a build table with 78 million of rows is a small data set? It might be considered smaller in the eye of the Oracle optimizer when the probe table is estimated to generate 770 million of rows. This is exactly what happened to me.

If this concept of parallel data distribution is still not clear for you and you want to understand what I have encountered and how I’ve managed to pull out myself from this nightmare then continuing reading this article might be worth the effort.

I was asked to create a big table based on a join with two other big tables so that doing this serially was practically impossible to complete in an acceptable execution time. I decided, therefore, to create it in parallel. After having enabled parallel DML and forced parallel DDL I launched the following create table statement with a DOP of 8 both for DML and DDL

SQL> create /*+ parallel(8) */
       table table_of_dates
       tablespace dtbs
       pctfree 0
    as
    select
        /*+ parallel(8)
          full(t)
          full(tr)
      */
       t.tr_tabl_id
     , t.table1_date_time
     , t.writing_date
     , min(tr.arrivl_date)
     , max(tr.arrivl_date)
   from
      table1 t
   left outer join table2 tr
   on t.tr_tabl_id = tr.tr_tabl_id
   join table3
   on t.order_id = table3.order_id
   and tr.status not in ('CANCELED')
   where t.writing_date <= to_date('17.06.2011', 'dd.mm.yyyy')
   and table3.order_type = ‘Broadcast’
   group by
         t.tr_tabl_id
       , t.table1_date_time
       , t.writing_date;

create /*+ parallel(8) */
*
ERROR at line 1:
ORA-12801: error signaled in parallel query server P013
ORA-01652: unable to extend temp segment by 128 in tablespace TEMP

As you can notice, it went with an ORA-01652 error. Below it the corresponding Real Time SQL Monitoring Report showing that the HASH JOIN operation at line 7 reaches 67G which is beyond the size of the current physical TEMP tablespace (64G) and hence the ORA-01652 error.

Parallel Execution Details (DOP=8, Servers Allocated=16)
SQL Plan Monitoring Details (Plan Hash Value=3645515647)
=====================================================================================
| Id |              Operation              |        Name | Execs |   Rows   | Temp  |
|    |                                     |             |       | (Actual) | (Max) |
=====================================================================================
|  0 | CREATE TABLE STATEMENT              |             |    17 |          |       |
|  1 |   PX COORDINATOR                    |             |    17 |          |       |
|  2 |    PX SEND QC (RANDOM)              | :TQ10003    |       |          |       |
|  3 |     LOAD AS SELECT                  |             |       |          |       |
|  4 |      HASH GROUP BY                  |             |       |          |       |
|  5 |       PX RECEIVE                    |             |       |          |       |
|  6 |        PX SEND HASH                 | :TQ10002    |     8 |          |       |
|  7 |         HASH JOIN                   |             |     8 |        0 |   67G |
|  8 |          PX RECEIVE                 |             |     8 |     626M |       |
|  9 |           PX SEND BROADCAST         | :TQ10001    |     8 |     626M |       |
| 10 |            HASH JOIN                |             |     8 |      78M |       |
| 11 |             PX RECEIVE              |             |     8 |     372K |       |
| 12 |              PX SEND BROADCAST      | :TQ10000    |     8 |     372K |       |
| 13 |               PX BLOCK ITERATOR     |             |     8 |    46481 |       |
| 14 |                INDEX FAST FULL SCAN | IDX_TABLE3_3|   182 |    46481 |       |
| 15 |             PX BLOCK ITERATOR       |             |     8 |      88M |       |
| 16 |              TABLE ACCESS FULL      | TABLE1      |   120 |      88M |       |
| 17 |          PX BLOCK ITERATOR          |             |     8 |     717M |       |
| 18 |           TABLE ACCESS FULL         | TABLE2      |   233 |     717M |       |
=====================================================================================

I stumped few minutes looking at the above execution plan and have finally decided to try a second create table with a reduced degree of parallelism (4 instead of 8) and here what I got

SQL> create /*+ parallel(4) */
       table table_of_dates
       tablespace dtbs
       pctfree 0
    as
    select
        /*+ parallel(4)
         full(t)
         full(tr)
      */
       t.tr_tabl_id
     , t.table1_date_time
     , t.writing_date
     , min(tr.arrivl_date)
     , max(tr.arrivl_date)
   from
      table1 t
   left outer join table2 tr
   on t.tr_tabl_id = tr.tr_tabl_id
   join table3
   on t.order_id = table3.order_id
   and tr.status not in ('CANCELED')
   where t.writing_date <= to_date('17.06.2011', 'dd.mm.yyyy')
   and table3.order_type = ‘Broadcast’
   group by
         t.tr_tabl_id
       , t.table1_date_time
       , t.writing_date;

Table created.
Elapsed: 00:31:42.29

The table has been this time successfully created within 32 minutes approximatively.

Before going to the next issue in the pipe, I wanted to understand why reducing the Degree Of Parallelism (DOP) from 8 to 4 made the create statement successful? The obvious thing I have attempted was to compare the 8 DOP execution plan with the 4 DOP one. The first plan has already been shown above. The second one is presented here below (reduced only to the information that is vital to the aim of this article):

Parallel Execution Details (DOP=4 , Servers Allocated=8)
SQL Plan Monitoring Details (Plan Hash Value=326881411)
=============================================================================================
| Id |                 Operation                  |       Name     | Execs |   Rows   |Temp |
|    |                                            |                |       | (Actual) |(Max)|
=============================================================================================
|  0 | CREATE TABLE STATEMENT                     |                |     9 |        4 |     |
|  1 |   PX COORDINATOR                           |                |     9 |        4 |     |
|  2 |    PX SEND QC (RANDOM)                     | :TQ10003       |     4 |        4 |     |
|  3 |     LOAD AS SELECT                         |                |     4 |        4 |     |
|  4 |      HASH GROUP BY                         |                |     4 |      75M |     |
|  5 |       PX RECEIVE                           |                |     4 |     168M |     |
|  6 |        PX SEND HASH                        | :TQ10002       |     4 |     168M |     |
|  7 |         HASH JOIN                          |                |     4 |     168M | 34G |
|  8 |          PX RECEIVE                        |                |     4 |     313M |     |
|  9 |           PX SEND BROADCAST                | :TQ10001       |     4 |     313M |     |
| 10 |            HASH JOIN                       |                |     4 |      78M |     |
| 11 |             BUFFER SORT                    |                |     4 |     186K |     |
| 12 |              PX RECEIVE                    |                |     4 |     186K |     |
| 13 |               PX SEND BROADCAST            | :TQ10000       |     1 |     186K |     |
| 14 |                TABLE ACCESS BY INDEX ROWID | TABLE3         |     1 |    46481 |     |
| 15 |                 INDEX RANGE SCAN           | IDX_ORDER_TYPE |     1 |    46481 |     |
| 16 |             PX BLOCK ITERATOR              |                |     4 |      88M |     |
| 17 |              TABLE ACCESS FULL             | TABLE1         |   115 |      88M |     |
| 18 |          PX BLOCK ITERATOR                 |                |     4 |     770M |     |
| 19 |           TABLE ACCESS FULL                | TABLE2         |   256 |     770M |     |
=============================================================================================

They don’t share exactly the same execution plan (they have two different plan hash values). The irritating question was: why, by halving down the degree of parallelism from 8 to 4 the same SQL statement necessitated almost half (34G) the amount of TEMP space and completed successfully?

The answer resides into the parallel distribution (PQ Distrib) method used by the parallel server sets to distribute their collected set of rows to the subsequent parallel server set (doing the hash join).

In the DOP 4 execution plan above we can see that we have 4 PX parallel server sets each one responsible of filling up one of the virtual TQ tables: TQ10000, TQ10001, TQ10002 and TQ10003. Here below is how to read the above execution plan:

  1.  The first PX1 set of slaves reads TABLE3 and broadcast its data set to the second PX2 set of slaves through the TQ10000 virtual table.
  2.  PX2 set reads TABLE1, hash join it with the data set it has received (TQ10000) from PX1 set and broadcast its result to the next parallel server
    set which is PX3 via the second TQ10001 virtual table.
  3. PX3 set of parallel slaves probes TABLE2 by parallel full scanning it and hash join it with the build result set (TQ10001) it has received from
    PX2 parallel set. This operation ends up by filling up the third virtual TQ table TQ10002 and by sending it to the next and last PX server PX4
    using a hash distribution.
  4. Finally, PX4 set of slaves will receive the TQ10002 data, hash group by it, fill the last virtual table (TQ10003) table and send it to the query
    coordinator (QC) which will end up by creating the table_of_dates table

That is a simple way of reading a parallel execution plan. By listing the above parallel operations I aimed to emphasize that data (HASH JOIN operation at line 10) produced by the parallel set of slaves PX2 is broadcasted (PX SEND BROADCAST operation at line 9) to the next parallel set PX3. And this means that each slave of PX2 set will pass every row it has received to every slave of the PX3 set. A typical reason to do such a distribution method is that the data set of the first row source (78M of rows in this case) is ”smaller” than the second row source to be joined with (770M of rows).

In order to make the picture clear let’s zoom around the hash join operation in the 4 DOP plan

Parallel Execution Details (DOP=4 , Servers Allocated=8)
======================================================================
| Id |        Operation    |       Name     | Execs |   Rows   |Temp |
|    |                     |                |       | (Actual) |(Max)|
======================================================================
|  7 |HASH JOIN            |                |     4 |     168M | 34G |
|  8 | PX RECEIVE          |                |     4 |     313M |     |
|  9 |  PX SEND BROADCAST  | :TQ10001       |     4 |     313M |     |
| 10 |   HASH JOIN         |                |     4 |      78M |     |
| 18 | PX BLOCK ITERATOR   |                |     4 |     770M |     |
| 19 |  TABLE ACCESS FULL  | TABLE2         |   256 |     770M |     |
======================================================================

The ”smaller” result set produced by the PX2 set of slaves at line 10 is 78M of rows. As far as Oracle decided to broadcast those 78M of rows towards PX3 set it has been duplicated as many times as there are slaves in PX3 set to receive the broadcasted data. As far as we have a DOP of 4 then this means that the HASH JOIN operation at line 7 has received 4*78 = 313M of rows (operation at line 8) that has been built and hashed in memory in order to be able to probe the result set produced by the second parallel server set coming from operations 18-19.

Multiply the DOP by 2, keep the same parallel distribution method and the same HASH JOIN operation will have to build and hash a table of 8*78 = 626M of rows. Which ultimately has required more than 67GB of TEMP space (instead of the initial DOP 4 34GB) as shown below:

Parallel Execution Details (DOP=8, Servers Allocated=16)
===================================================================
| Id |     Operation     |        Name | Execs |   Rows   | Temp  |
|    |                   |             |       | (Actual) | (Max) |
===================================================================
|  7 |HASH JOIN          |             |     8 |        0 |   67G |
|  8 | PX RECEIVE        |             |     8 |     626M |       |
|  9 |  PX SEND BROADCAST| :TQ10001    |     8 |     626M |       |
| 10 |   HASH JOIN       |             |     8 |      78M |       |
| 17 | PX BLOCK ITERATOR |             |     8 |     717M |       |
| 18 |  TABLE ACCESS FULL| TABLE2      |   233 |     717M |       |
===================================================================

Now that I know from where the initial ORA-01652 error is coming from, changing the parallel distribution method from BROADCAST to HASH might do the job without requiring more than 64GB of TEMP space. A simple way to accomplish this task is to hint the parallel select to use the desired parallel distribution method. From the outline of the successful DOP 4 execution plan I took the appropriate hint (pq_distribute(alias hash hash)), adapted it and issued the following create table:

SQL> create /*+ parallel(8) */
       table table_of_dates
       tablespace dtbs
       pctfree 0
    as
    select
        /*+ parallel(8)
          full(t)
          full(tr)
	  pq_distribute(@"SEL$E07F6F7C" "T"@"SEL$2" hash hash)
          px_join_filter(@"SEL$E07F6F7C" "T"@"SEL$2")
          pq_distribute(@"SEL$E07F6F7C" "TR"@"SEL$1" hash hash)
          px_join_filter(@"SEL$E07F6F7C" "TR"@"SEL$1")
      */
       t.tr_tabl_id
     , t.table1_date_time
     , t.writing_date
     , min(tr.arrivl_date)
     , max(tr.arrivl_date)
   from
      table1 t
   left outer join table2 tr
   on t.tr_tabl_id = tr.tr_tabl_id
   join table3
   on t.order_id = table3.order_id
   and tr.status not in ('CANCELED')
   where t.writing_date <= to_date('17.06.2011', 'dd.mm.yyyy')
   and table3.order_type = ‘Broadcast’
   group by
         t.tr_tabl_id
       , t.table1_date_time
       , t.writing_date;	   

Table created.
Elapsed: 00:12:46.33

And the job has been done in less than 13 minutes instead of the initial 33 minutes with DOP 4.

The new execution plan is:

SQL Plan Monitoring Details (Plan Hash Value=5257928)
====================================================================================
| Id |              Operation              |        Name |Execs |   Rows   | Temp  |
|    |                                     |             |      | (Actual) | (Max) |
====================================================================================
|  0 | CREATE TABLE STATEMENT              |             |   17 |        8 |       |
|  1 |   PX COORDINATOR                    |             |   17 |        8 |       |
|  2 |    PX SEND QC (RANDOM)              | :TQ10004    |    8 |        8 |       |
|  3 |     LOAD AS SELECT                  |             |    8 |        8 |       |
|  4 |      HASH GROUP BY                  |             |    8 |      75M |   10G |
|  5 |       HASH JOIN                     |             |    8 |     168M |       |
|  6 |        JOIN FILTER CREATE           | :BF0000     |    8 |      78M |       |
|  7 |         PX RECEIVE                  |             |    8 |      78M |       |
|  8 |          PX SEND HASH               | :TQ10002    |    8 |      78M |       |
|  9 |           HASH JOIN BUFFERED        |             |    8 |      78M |    3G |
| 10 |            JOIN FILTER CREATE       | :BF0001     |    8 |    46481 |       |
| 11 |             PX RECEIVE              |             |    8 |    46481 |       |
| 12 |              PX SEND HASH           | :TQ10000    |    8 |    46481 |       |
| 13 |               PX BLOCK ITERATOR     |             |    8 |    46481 |       |
| 14 |                INDEX FAST FULL SCAN | IDX_TABLE3_3|  181 |    46481 |       |
| 15 |            PX RECEIVE               |             |    8 |      88M |       |
| 16 |             PX SEND HASH            | :TQ10001    |    8 |      88M |       |
| 17 |              JOIN FILTER USE        | :BF0001     |    8 |      88M |       |
| 18 |               PX BLOCK ITERATOR     |             |    8 |      88M |       |
| 19 |                TABLE ACCESS FULL    | TABLE1      |  121 |      88M |       |
| 20 |        PX RECEIVE                   |             |    8 |     770M |       |
| 21 |         PX SEND HASH                | :TQ10003    |    8 |     770M |       |
| 22 |          JOIN FILTER USE            | :BF0000     |    8 |     770M |       |
| 23 |           PX BLOCK ITERATOR         |             |    8 |     770M |       |
| 24 |            TABLE ACCESS FULL        | TABLE2      |  245 |     770M |       |
====================================================================================

Changing the parallel distribution method from broadcast to hash-hash has not only reduced the TEMP space usage but has halved the execution time of the create table.

Conclusion
Watch carefully the degree of parallelism you are going to use. In case of a broadcast distribution method for relatively big ”smaller result set” you might end up by devouring a huge amount of TEMP space and failing to succeed with the ORA-01652: unable to extend temp segment by 128 in tablespace TEMP. Hopefully, with the arrival of the 12c Adaptive Distribution Method and the new HYBRID HASH parallel distribution method, the STATISTIC COLLECTOR operation placed below the distribution method will operate a dynamic switch from a BROADCAST distribution to a HASH distribution whenever the number of rows to be distributed exceeds a threshold which is 2*DOP. With my initial DOP of 8 I would have had a threshold of 16 which is largely below the 78 million of rows that I am due to distribute further up.

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February 14, 2015

Resumable time out

Filed under: Tuning — hourim @ 9:47 am

I was trying to create a very big table through a create table as select from a join between two huge tables forcing both parallel DDL and parallel QUERY and was mainly concerned about the amount of temp space the hash join between those two big tables will require to get my table smoothly created. I said hash join because my first attempt to create this table used a nested loop join which was driving a dramatic 100 million starts of table access by index rowid since more than 10 hours when I decided to kill the underlying session. The create table being a one-shot process I decided to hint the optimizer so that it will opt for a hash join operation instead of the initial nested loop. But my concern has been transferred from a long running non finishing SQL statement to how much my create table will need of TEMP space in order to complete successfully.

I launched the create table and started monitoring it with Tanel Poder snapper script, v$active_session_history and the Real Time SQL Monitoring feature (RTSM). This is what the monitoring was showing

SQL> select 
          event
	 ,count(1) 
      from 
          v$active_session_history 
      where sql_id = '0xhm7700rq6tt' 
      group by event 
      order by 2 desc;


EVENT                                                              COUNT(1)
---------------------------------------------------------------- ----------
direct path read                                                       3202
direct path write temp                                                 1629
                                                                        979
db file scattered read                                                    8
CSS initialization                                                        7
CSS operation: query                                                      1

However few minutes later Tanel Poder snapper started showing the following dominant wait event

SQL> @snapper ash 5 1 all

----------------------------------------------------------------------------------------------------
Active% | INST | SQL_ID          | SQL_CHILD | EVENT                               | WAIT_CLASS
----------------------------------------------------------------------------------------------------
   800% |    1 | 0xhm7700rq6tt   | 0         | statement suspended, wait error to  | Configuration

--  End of ASH snap 1, end=2015-02-12 09:12:57, seconds=5, samples_taken=45

Event which keeps incrementing in v$active_session_history as shown below:

EVENT                                                              COUNT(1)
---------------------------------------------------------------- ----------
statement suspended, wait error to be cleared                          7400
direct path read                                                       3202
direct path write temp                                                 1629
                                                                        979
db file scattered read                                                    8
CSS initialization                                                        7
CSS operation: query                                                      1

7 rows selected.

SQL> /

EVENT                                                              COUNT(1)
---------------------------------------------------------------- ----------
statement suspended, wait error to be cleared                          7440
direct path read                                                       3202
direct path write temp                                                 1629
                                                                        979
db file scattered read                                                    8
CSS initialization                                                        7
CSS operation: query                                                      1

7 rows selected.

SQL> /

EVENT                                                              COUNT(1)
---------------------------------------------------------------- ----------
statement suspended, wait error to be cleared                          7480
direct path read                                                       3202
direct path write temp                                                 1629
                                                                        979
db file scattered read                                                    8
CSS initialization                                                        7
CSS operation: query                                                      1

EVENT                                                              COUNT(1)
---------------------------------------------------------------- ----------
statement suspended, wait error to be cleared                         11943
direct path read                                                       3202
direct path write temp                                                 1629
                                                                        980
db file scattered read                                                    8
CSS initialization                                                        7
CSS operation: query                                                      1

The unique wait event that was incrementing is that bizarre statement suspended, wait error to be cleared. The session from which I launched the create table statment was hanging without reporting any error.

As far as I was initially concerned by the TEMP space I checked the available space there

SQL> select 
       tablespace_name,
       total_blocks,
       used_blocks,
       free_blocks
     from v$sort_segment;

TABLESPACE_NAME                 TOTAL_BLOCKS USED_BLOCKS FREE_BLOCKS
------------------------------- ------------ ----------- -----------
TEMP                                 8191744     8191744           0    

No available free space left in TEMP tablespace

The RTSM of the corresponding sql_id was showing the following

=====================================================================================
| Id    |              Operation              |        Name        |  Rows   | Temp |
|       |                                     |                    |(Actual) |      |
=====================================================================================
|     0 | CREATE TABLE STATEMENT              |                    |       0 |      |
|     1 |   PX COORDINATOR                    |                    |         |      |
|     2 |    PX SEND QC (RANDOM)              | :TQ10003           |         |      |
|     3 |     LOAD AS SELECT                  |                    |         |      |
|     4 |      HASH GROUP BY                  |                    |         |      |
|     5 |       PX RECEIVE                    |                    |         |      |
|     6 |        PX SEND HASH                 | :TQ10002           |         |      |
|  -> 7 |         HASH JOIN                   |                    |       0 |  66G | --> spot this 
|       |                                     |                    |         |      |
|       |                                     |                    |         |      |
|       |                                     |                    |         |      |
|     8 |          PX RECEIVE                 |                    |    626M |      |
|     9 |           PX SEND BROADCAST         | :TQ10001           |    626M |      |
|    10 |            HASH JOIN                |                    |     78M |      |
|    11 |             PX RECEIVE              |                    |    372K |      |
|    12 |              PX SEND BROADCAST      | :TQ10000           |    372K |      |
|    13 |               PX BLOCK ITERATOR     |                    |   46481 |      |
|    14 |                INDEX FAST FULL SCAN | INDX_12453656_TABL |   46481 |      |
|    15 |             PX BLOCK ITERATOR       |                    |     88M |      |
|    16 |              TABLE ACCESS FULL      | TABL1              |     88M |      |
|       |                                     |                    |         |      |
| -> 17 |          PX BLOCK ITERATOR          |                    |    682M |      |
| -> 18 |           TABLE ACCESS FULL         | TABLE123456        |    682M |      |
|       |                                     |                    |         |      |
=====================================================================================

The HASH JOIN operation at line 7 has already gone above the available TEMP tablespace size which is 64G. I was wondering then, why my session has not been stopped with an ORA-01652 error instead of hanging there and letting the wait event statement suspended, wait error to be cleared popping up in my different monitoring tools.

After few minutes of googling tuning steps I ended up by executing the following query

SQL>select
        coord_session_id
       ,substr(sql_text, 1,10)
       ,error_msg
    from dba_resumable;

SESSION_ID  SQL        ERROR_MSG
----------- ---------  -------------------------------------------------------------------
146         create /*+  ORA-01652: unable to extend temp segment by 128 in tablespace TEMP

This 146 SID is my create table session! It is in error but hanging and waiting on that statement suspend wait event.

After few minutes of googling tuning again I ended up by checking the following parameter:

SQL> sho parameter resumable

NAME                           TYPE                             VALUE
------------------------------ -------------------------------- ------
resumable_timeout              integer                          9000

Notice now how I finally have made a link between the title and the purpose of this article: resumable_timeout. The Oracle documentation states that when this parameter is set to a nonzero value the resumable space allocation might kicks in.

Put is simply what happens in my case is that since this parameter has been set to 9000 minutes and since my create table as select has encountered an ORA-01652 temp tablespace error, Oracle decided to put my session in a resumable state until someone will point out that space shortage,correct the situation and allow the session to continue its normal processing

The next step I did is to abort my session from its resumable state

SQL> exec dbms_resumable.abort(146);
 
PL/SQL procedure successfully completed.

Unfortunately I waited a couple of minutes in front of my sqlplus session waiting for it to resume without success so that I decided to use the brute force

ERROR:
ORA-03113: end-of-file on communication channel
Process ID: 690975
Session ID: 146 Serial number: 3837

Summary
I would suggest to do not set the resumable_timeout parameter to a nonzero value. Otherwise you will be hanging on a statement suspended, wait error to be cleared event during an amount of time indicated by this resumable_timeout parameter looking around for what is making your SQL statement hanging.

Footnote
Here below few extra words from Oracle documentation about Resumable space allocation

“Resumable space allocation avoids headaches and saves time by suspending, instead
of terminating, a large database operation requiring more disk space than is currently
available. While the operation is suspended, you can allocate more disk space on
the destination tablespace or increase the quota for the user. Once the low space
condition is addressed, the large database operation automatically picks up where it
left off.
As you might expect, the statements that resume are known as resumable statements.
The suspended statement, if it is part of a transaction, also suspends the transaction.
When disk space becomes available and the suspended statement resumes, the
transaction can be committed or rolled back whether or not any statements in
the transactions were suspended. The following conditions can trigger resumable
space allocation:
■ Out of disk space in a permanent or temporary tablespace
■ Maximum extents reached on a tablespace
■ User space quota exceeded
You can also control how long a statement can be suspended. The default time
interval is two hours, at which point the statement fails and returns an error message
to the user or application as if the statement was not suspended at all.
There are four general categories of commands that can be resumable:
(1) SELECT statements, (2) DML commands, (3) SQL*Loader operations, and
(4) DDL statements that allocate disk space.”

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