Performance of query using view - what is happening here

Question

Hi,
I can't explain the difference in performance between two queeries.
For a datawarehouse I have 3 tables from 3 different sources, named source1, source2 and
source3 they all have the identical columns:
client_key
,client_revenue
source1 has 90.000.000 rows, source2 1.000.000 rows and source3 50.000 rows
I also made a view say, all_clients which is the union of the 3 tables plus a constant column called 'source'
which corresponds to the table_name.
If I run a query which shows the number of records it takes 15-20 minutes:
select source,count(*)
from all_clients
group by source.
If i run the following query it takes about 5 minutes!
select 'source1',count(*)
from source1
union
select 'source2',count(*)
from source2
union
select 'source3',count(*)
from source3.
What makes the difference?

Answer 1

Hmmm... Interesting. In my small example things seem pretty similar. Have you done the explain plans?
An observation is that you are using a UNION rather than a UNION ALL which would be better as you may be incurring an unnecessary SORT UNIQUE.
create table tab1 as(select object_id, object_type from all_objects);
create table tab2 as(select object_id, object_type from all_objects);
create table tab3 as(select object_id, object_type from all_objects);
analyze table tab1 estimate statistics;
analyze table tab2 estimate statistics;
analyze table tab3 estimate statistics;
create view v_tab123 as(select 'source1' source,count(*) cnt
from tab1
union
select 'source2',count(*)
from tab2
union
select 'source3',count(*)
from tab3);
select 'source1' source,count(*) cnt
from tab1
union
select 'source2',count(*)
from tab2
union
select 'source3',count(*)
from tab3;
Operation     Object Name     Rows     Bytes     Cost     TQ     In/Out     PStart     PStop
SELECT STATEMENT Hint=CHOOSE          3           180                     
SORT UNIQUE          3           180                     
UNION-ALL                                        
SORT AGGREGATE          1           60                     
TABLE ACCESS FULL     TAB1     38 K          10                     
SORT AGGREGATE          1           60                     
TABLE ACCESS FULL     TAB2     38 K          10                     
SORT AGGREGATE          1           60                     
TABLE ACCESS FULL     TAB3     38 K          10                     
-- Union
select source, cnt from(
select 'source1' source,count(*) cnt
from tab1
union
select 'source2',count(*)
from tab2
union
select 'source3',count(*)
from tab3)
Operation     Object Name     Rows     Bytes     Cost     TQ     In/Out     PStart     PStop
SELECT STATEMENT Hint=CHOOSE          3           180                     
VIEW          3      54      180                     
SORT UNIQUE          3           180                     
UNION-ALL                                        
SORT AGGREGATE          1           60                     
TABLE ACCESS FULL     TAB1     38 K          10                     
SORT AGGREGATE          1           60                     
TABLE ACCESS FULL     TAB2     38 K          10                     
SORT AGGREGATE          1           60                     
TABLE ACCESS FULL     TAB3     38 K          10                     
-- Union ALL
select source, cnt from(
select 'source1' source,count(*) cnt
from tab1
union ALL
select 'source2',count(*)
from tab2
union ALL
select 'source3',count(*)
from tab3)
Operation     Object Name     Rows     Bytes     Cost     TQ     In/Out     PStart     PStop
SELECT STATEMENT Hint=CHOOSE          3           180                     
VIEW          3      54      180                     
SORT UNIQUE          3           180      <<<<============== Unnecessary           
UNION-ALL                                        
SORT AGGREGATE          1           60                     
TABLE ACCESS FULL     TAB1     38 K          10                     
SORT AGGREGATE          1           60                     
TABLE ACCESS FULL     TAB2     38 K          10                     
SORT AGGREGATE          1           60                     
TABLE ACCESS FULL     TAB3     38 K          10                     
analyze table tab1 delete statistics;
analyze table tab2 delete statistics;
analyze table tab3 delete statistics;
Now with RBO - the SORT UNIQUE goes away for the above query.
Operation     Object Name     Rows     Bytes     Cost     TQ     In/Out     PStart     PStop
SELECT STATEMENT Hint=CHOOSE                                        
VIEW                                        
UNION-ALL                                        
SORT AGGREGATE                                        
TABLE ACCESS FULL     TAB1                                   
SORT AGGREGATE                                        
TABLE ACCESS FULL     TAB2                                   
SORT AGGREGATE                                        
TABLE ACCESS FULL     TAB3