Table access by index rowid taking more time

Question

Hi All
I've a query like
update tab1
  set col1 = ( select col2 from
                       tab2 
                where tab1.id = tab2.id) table 1 has arnd 10,000 rows
table 2 has arnd 1,700,000 rows and has a primay key on column id.
This query is taking around 20 secs to execute. I checked the xplan and most of time taken for table access by index rowid.
Could you please suggest what can be the reason for this. (Can it be the clustering factor or something else)
I checked the stats for the tab2, its just three days old.
Regards
Ashwani

Answer 1

>
table 1 has arnd 10,000 rows
table 2 has arnd 1,700,000 rows and has a primay key on column id.
This query is taking around 20 secs to execute. I checked the xplan and most of time taken for table access by index rowid.
Could you please suggest what can be the reason for this. (Can it be the clustering factor or something else)
I checked the stats for the tab2, its just three days old.
>
If you checked the xplan why haven't you posted it so we can look at it? Then we could see what table is being accessed by index rowid. Presumably it is table 2 but we the plan would eliminate the need to make assumptions.
The clustering factor could be a factor. You haven't told us how table1 is being accessed. All rows are being updated so a full table scann is most likely but again the plan would actually show the access.
Did you query the dictionary to see what the clustering factor is? Post the results of that
SQL> select index_name, leaf_blocks, avg_leaf_blocks_per_key, avg_data_blocks_per_key, clustering_factor, distinct_keys
2 from dba_indexes
3 where owner = 'schema'
4 and index_name in ('index_b','index_a');