Oracle + compression + temp space usage

Question

I have a Oracle SQL query that consumes lot of temp space during "GROUP BY" operation. Underlying table has 20 billion records (18 GB). I'm planning to compress that table and see if it would help in reducing the temp space that it would occupy. I doubt it may not but has someone experienced it ?

Thanks in advance..

Answer 1

Basic table compression does not reduce temporary tablespace usage. It's difficult to prove something does not exist, here are my reasons:

1. Such a feature is not mentioned in the manuals or advertised as a feature.
2. I can't think of an easy way to implement it. Basic table compression is not deterministic. The same values can be represented in many different ways, therefore the data must be uncompressed before it can be meaningfully joined with other data.
3. A simple test case does not show any affect.

This example show a highly compressed table being grouped. Adding or removing the compress option does not change the amount of temporary space.

--Create a table and add data.
drop table table1;
create table table1(
    c1 char(100), c2 char(100), c3 char(100), c4 char(100), c5 char(100),
    c6 char(100), c7 char(100), c8 char(100), c9 char(100), c10 char(100)
) compress; -- Remove "compress" and re-run to compare.  Results will not change.
insert /*+ append */ into table1
select level,level,level,level,level,level,level,level,level,level
from dual connect by level <= 100000;
commit;

--There is about 95MB of data.
select 100000*100*10/1024/1024 mb from dual;

--Segment size is 13MB.
select bytes/1024/1024 from dba_segments where segment_name = 'TABLE1';

--Create a new table using a large GROUP BY.
drop table table2;
create table table2 as
select c1,c2,c3,c4,c5,c6,c7,c8,c9,c10
from table1
group by c1,c2,c3,c4,c5,c6,c7,c8,c9,c10;

--In a separate session, measure the temporary tablespace usage.
--The value tops out at 89MB.
select bytes/1024/1024 mb from dba_segments where segment_name = 'TABLE1';