Gathering SQL execution details

Oracle Tips by Burleson Consulting
Match 5, 2008

In addition to TKPROF (SQL*Trace) and the SQL*Plus "set autotrace on" command, we see new tools for getting additional SQL execution information.

Staring with Oracle, we see several exciting new execution plan columns and you can ruin specialized queries to see estimates of resource usage associated with specific steps of Oracle SQL execution:

CPU_COST - The CPU cost of the operation as estimated by the optimizer's cost-based approach. For statements that use the rule-based approach, this column is null. The value of this column is proportional to the number of machine cycles required for the operation.
IO_COST - The I/O cost of the operation as estimated by the optimizer's cost-based approach. For statements that use the rule-based approach, this column is null. The value of this column is proportional to the number of data blocks read by the operation
TEMP_SPACE - The temporary space, in bytes, used by the operation as estimated by the optimizer's cost-based approach. For statements that use the rule-based approach, or for operations that don't use any temporary space, this column is null.

In Oracle 10g, also see gather_plan_statistics hint tips where you can gather estimated rows vs. actual rows. Oracle claims to use this information to make more intelligent choices of execution plans, but there is a serious problem with this approach.

1: The CBO has no a-priori knowledge of the data buffer contents: Because of this shortcoming, the CBO cannot know if the data blocks are already in the RAM data buffers.

2: CPU costs depend upon system load: The CPU costs associated with servicing an Oracle query depend upon the server load, and CPU costs are generally not important unless the entire Oracle instance is using excessive CPU resources.

3: Temporary segment space is transient: Even though the CBO estimates the TEMP_SPACE costs at optimization time, this execution plan will become permanent for the SQL query until the executable is flushed from the library cache. Hence, obsolete execution plans may be used to execute a query.

Oracle also has two new views to show active work area space, the v$sql_workarea and the v$sql_workarea_active views. The v$sql_workarea_active view will display all of the work areas that are currently executing in the instance. Note that small sorts (under 65,535 bytes) are excluded from the view. The v$sql_workarea_active view can be used to quickly monitor the size of all large active work areas.

workarea.sql

select
   to_number(decode(SID, 65535, NULL, SID)) sid,
   operation_type              OPERATION,
   trunc(WORK_AREA_SIZE/1024) WSIZE,
   trunc(EXPECTED_SIZE/1024)   ESIZE,
   trunc(ACTUAL_MEM_USED/1024) MEM,
   trunc(MAX_MEM_USED/1024)    "MAX MEM",
   number_passes               PASS
from
   v$sql_workarea_active
order by
   1,2;

Here is a sample listing from this script, showing the work area estimated vs. actual size, plus the memory used for the specific operation. This is a great way to see the actual RAM used in a sort or hash join operation.

SID OPERATION             WSIZE     ESIZE       MEM   MAX MEM PASS
--- --------------------- ----- --------- --------- --------- ----
27 GROUP BY (SORT)          73        73        64        64    0
44 HASH-JOIN              3148      3147      2437      6342    1
71 HASH-JOIN             13241     19200     12884     34684    1

For more details on gathering run-time SQL execution statistics see my book "Oracle Tuning: The Definitive Reference".

This output shows that session 44 is running a hash join whose work area is running in one-pass mode. This work area is currently using 2 megabytes of PGA memory and in the past, has used up to 6.5 megabytes.

This view is very useful for assessing the current memory operations within Oracle. You can use the SID column to join into the v$process and v$session views for additional information about each task.

This script is good for finding the top ten work areas currently allocated in the PGA RAM:

select
   c.sql_text,
   w.operation_type,
   top_ten.wasize
from
   (Select *
    from
      (Select workarea_address, actual_mem_used wasize
       from v$sql_workarea_active
order by actual_mem_used)
where
   ROWNUM <=10) top_ten,
   v$sql_workarea w,
   v$sql c
where
   w.workarea_address=top_ten.workarea_address
and
   c.address=w.address
and
   c.child_number = w.child_number
and
   c.hash_value=w.hash_value;

This script is good for finding the percentage of RAM that is over and under allocated:

select
   total_used,
   under*100/(total_used+1) percent_under_use,
   over*100/(total_used+1) percent_over_used
from
   (Select
    sum(case when expected_size > actual_mem_used
      then actual_mem_used else 0 end) under,
    sum(case when expected_size<> actual_mem_used
       then actual_mem_used else 0 end) over,
    sum(actual_mem_used) total_used
from
   v$sql_workarea_active
where policy='AUTO') usage;

For full scripts, download the Oracle script collection.

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