Oracle 11g how to estimate needed TEMP tablespace?

Question

We do an initial bulk load of some tables (both, source and target are Oracle 11g). The process is as follows: 1. truncate, 2. drop indexes (the PK and a unique index), 3. bulk insert, 4. create indexes (again the PK and the unique index). Now I got the following error:

alter table TARGET_SCHEMA.MYBIGTABLE
add constraint PK_MYBIGTABLE primary key (MYBIGTABLE_PK)
ORA-01652: unable to extend temp segment by 128 in tablespace TEMP

So obviously TEMP tablespace is to small for PK creation (FYI the table has 6 columns and about 2.2 billion records). So I did this:

explain plan for
select line_1,line_2,line_3,line_4,line_5,line_6,count(*) as cnt
from SOURCE_SCHEMA.MYBIGTABLE
group by line_1,line_2,line_3,line_4,line_5,line_6;

select * from table( dbms_xplan.display );
/*
-----------------------------------------------------------------------------------------------
| Id  | Operation          | Name             | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |                  |  2274M|    63G|       |    16M  (2)| 00:05:06 |
|   1 |  HASH GROUP BY     |                  |  2274M|    63G|   102G|    16M  (2)| 00:05:06 |
|   2 |   TABLE ACCESS FULL| MYBIGTABLE       |  2274M|    63G|       |   744K  (7)| 00:00:14 |
-----------------------------------------------------------------------------------------------
*/

Is this how to tell how much TEMP tablespace will be needed for PK creation (102 GB in my case)? Or would you make the estimate differently?

Additional: The PK only exists on the target system. But fair point, so I run your query on target PK:

explain plan for
select MYBIGTABLE_PK 
from TARGET_SCHEMA.MYBIGTABLE
group by MYBIGTABLE_PK ;

-------------------------------------------------------------------------------------------
| Id  | Operation          | Name                 | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |                      |     1 |    13 |     3  (34)| 00:00:01 |
|   1 |  HASH GROUP BY     |                      |     1 |    13 |     3  (34)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| MYBIGTABLE           |     1 |    13 |     2   (0)| 00:00:01 |
-------------------------------------------------------------------------------------------

So how would I have to read this now?

Answer 1

This is a good question.

First, If you create the following primary key

alter table TARGET_SCHEMA.MYBIGTABLE 
     add constraint PK_MYBIGTABLE primary key (MYBIGTABLE_PK)

then you should query

explain plan for 
     select PK_MYBIGTABLE 
     from SOURCE_SCHEMA.MYBIGTABLE 
     group by PK_MYBIGTABLE 

To get an estimate (make sure you gather stats exec dbms_stats.gather_table_stats('SOURCE_SCHEMA','MYBIGTABLE').

Second , you can query V$TEMPSEG_USAGE to see how much temp blocks were consumed before you got thrown and v$session_longops to see how much of the total process you finished.

Oracle docs suggests creating a dedicated temp tablespace for the process to not disturb any other operations.

Please post an edit if you find a more accurate solution.