I see that Tom Kyte has found a nasty little bug waiting to trap a few unlucky people as they patch to 10.2.0.4, or upgrade to 11g.
By now, you should probably be using the pga_aggregate_target rather than the sort_area_size to control the memory allocation for sorting – especially from 10g onwards – but you may still have a few special tasks in big batch jobs where you have code that switches the workarea_size_policy to manual and sets a specific (large) sort_area_size. Perhaps something like:
alter session set workarea_size_policy = manual; alter session set sort_area_size = 104857600; --execute big job here alter session set workarea_size_policy = auto;
If this is the case, be careful when you patch to 10.2.0.4 or upgrade to 22.214.171.124, as the code may suddenly slow down quite dramatically.
There seems to be a bug with this particular ‘alter session’ command (and possibly with others – I haven’t yet tested) that “delays” the change. It’s quite easy to see this if you enable event 10032 to get the “sort parameters” dump. Here’s a sample of code I’ve run against a small 11g system (126.96.36.199) to demonstrate the issue.
create table t1 as with generator as ( select --+ materialize rownum id from all_objects where rownum <= 3000 ) select rownum id, lpad(rownum,10,'0') small_vc, lpad(rownum,100,'0') big_vc from generator v1, generator v2 where rownum <= 10000 ; -- gather statistics here
I’ve created a table with 10,000 rows and a couple of character columns that I’m now going to select and sort. The small_vc column is only 10 characters, so I shouldn’t need more than a couple of hundred KB to select and sort this column; and the big_vc column is only 100 bytes, so 4MB should be plenty to select and sort that column:
alter session set events '10032 trace name context forever'; alter session set workarea_size_policy = manual; alter session set sort_area_size = 262144; -- first the small one select count(*) from ( select /*+ qb_name(ordered) no_merge no_eliminate_oby(@ordered) */ small_vc from t1 order by small_vc ) ; -- then the big one alter session set sort_area_size = 4194304; select count(*) from ( select /*+ qb_name(ordered) no_merge no_eliminate_oby(@ordered) */ big_vc from t1 order by big_vc ) ;
I’ve used the /*+ no_merge */ hint in the SQL, with an inline view that contains the select and “order by” so that I can force Oracle to sort the data but only output a single row. Then I’ve used the /*+ no_eliminate_oby() */ hint to make sure that the optimizer doesn’t eliminate the (apparently) redundant “order by” clause in the in-line view (a trick that it learnt to do in 10gR2 – see the notes towards the end of this article).
The trace files dumped by event 10032 show the following results.
From the first query:
---- Sort Parameters ------------------------------ sort_area_size 65536 sort_area_retained_size 65536 sort_multiblock_read_count 1 max intermediate merge width 3 ---- Sort Statistics ------------------------------ Initial runs 1 Input records 10000 Output records 10000 Disk blocks 1st pass 20 Total disk blocks used 22 Total number of comparisons performed 10103 Comparisons performed by in-memory sort 10099 Comparisons while searching for key in-memory 4 Temp segments allocated 1 Extents allocated 1 Uses version 2 sort Uses asynchronous IO
Despite my call to set the sort_area_size to 256KB, the operation has used 64KB (the system default), and dumped about 160KB of data to disk.
Then, for the second query:
---- Sort Parameters ------------------------------ sort_area_size 262144 sort_area_retained_size 262144 sort_multiblock_read_count 2 max intermediate merge width 7 ---- Sort Statistics ------------------------------ Initial runs 1 Input records 10000 Output records 10000 Disk blocks 1st pass 134 Total disk blocks used 136 Total number of comparisons performed 10103 Comparisons performed by in-memory sort 10099 Comparisons while searching for key in-memory 4 Temp segments allocated 1 Extents allocated 2 Uses version 2 sort Uses asynchronous IO
I set the sort_area_size to 4MB, and the operation has used 256KB – which is the value I set on the previous sort. We can also see that we’ve dumped about 1.1MB of data to disk.
So, if you’ve got code that tries to set the sort_area_size before running a big job, it’s possible that that job will run with the wrong value for the parameter. The effects could be dire.
Fortunately the workaround is simple (and carries no side-effects) – just edit the code to do the ‘alter session’ twice in a row. The second call will ensure that the value you want actually gets used.