Often, to evaluate a SQL, we need to examine how it performed in the past.
Few things are important here:
1. Average elapsed time
2. Buffer gets per execution
3. Frequency of execution (how many times it executes on an hourly/daily basis)
The following script can be used to generate this information from AWR. The script assumes a RAC database.
In addition to basic information like snap time, instance id, plan hash and schema name, the following information is also generated:
SQL_PROFILE – name of the SQL profile is one was used
ROWS_DELTA – total number of rows fetched by all executions within the snap window
BUFFER_GETS_DELTA — total block gets within the window for the SQL
EXECUTIONS_DELTA — total number of executions within the window for the SQL
SECS_PER_EXEC — average time per execution within the window for the SQL
GETS_PER_EXEC — average block gets per execution; indicates approximately how heavy the query is
EXECS_THIS_DAY — total daily executions of this query
GETS_THIS_DAY — total daily buffer gets from this query
Script
--
-- sqlhist.sql
-- Parameters
-- 1. Number of days to look back
-- 2. SQL ID
-- Purpose: Generate the sql performance history from AWR
--
set verify on
set echo on
set lines 250
set head on
set tab off
col sql_profile format a20 wrap
col execs_this_day format 999,999,999
col gets_this_day format 999,999,999,999
WITH snaps AS
(SELECT instance_number, snap_id
FROM dba_hist_snapshot
WHERE begin_interval_time > SYSDATE - &days
)
SELECT /*artoftuning.net*/
rs.*,
SUM (executions_delta) OVER (PARTITION BY TRUNC (TO_DATE (snap_time, 'dd-mon-rr hh24:mi'))) AS execs_this_day,
SUM (buffer_gets_delta) OVER (PARTITION BY TRUNC (TO_DATE (snap_time, 'dd-mon-rr hh24:mi'))) AS gets_this_day
FROM (SELECT --snap_id,
(SELECT TO_CHAR (MIN (begin_interval_time),
'dd-mon-rr hh24:mi')
FROM dba_hist_snapshot
WHERE snap_id = s.snap_id
AND instance_number = s.instance_number) snap_time,
instance_number inst_id,
parsing_schema_name schema,
sql_id,
sql_profile,
plan_hash_value plan_hash,
rows_processed_delta rows_delta,
buffer_gets_delta,
executions_delta,
ROUND (elapsed_time_delta / 1000000 / DECODE (executions_delta, 0, 1, executions_delta), 2) secs_per_exec,
ROUND (buffer_gets_delta / DECODE (executions_delta, 0, 1, executions_delta)) gets_per_exec
FROM dba_hist_sqlstat s
WHERE sql_id = '&sql_id'
AND (instance_number, snap_id) IN (SELECT /*+no_unnest*/ instance_number, snap_id FROM snaps)
) rs
ORDER BY TO_DATE (snap_time, 'dd-mon-rr hh24:mi') DESC
/
CLEAR COLUMNS
-- end of script
Sample Output
Enter value for days: 1
old 4: WHERE begin_interval_time > SYSDATE - &days )
new 4: WHERE begin_interval_time > SYSDATE - 1 )
Enter value for sql_id: jjqmxszma79gi
old 33: WHERE sql_id = '&sql_id'
new 33: WHERE sql_id = 'jjqmxszma79gi'
SNAP_TIME INST_ID SCHEMA SQL_PROFILE PLAN_HASH ROWS_DELTA BUFFER_GETS_DELTA EXECUTIONS_DELTA SECS_PER_EXEC GETS_PER_EXEC EXECS_THIS_DAY GETS_THIS_DAY
--------------- ------- --------------------- ---------- ---------- ----------------- ---------------- ------------- ------------- -------------- -------------
05-jan-15 12:00 1 MARKETING 2909821187 0 0 0 0 0 4 3490638
05-jan-15 12:00 1 MARKETING 3363719406 11 968013 1 588.2 968013 4 3490638
05-jan-15 12:00 1 MARKETING 701902966 0 0 0 0 0 4 3490638
05-jan-15 08:00 1 MARKETING 3363719406 1 967893 1 1320.4 967893 4 3490638
05-jan-15 08:00 1 MARKETING 701902966 0 0 0 0 0 4 3490638
05-jan-15 08:00 1 MARKETING 2909821187 0 0 0 0 0 4 3490638
05-jan-15 04:00 1 MARKETING 3363719406 0 951896 1 729.1 951896 4 3490638
05-jan-15 04:00 1 MARKETING 701902966 0 0 0 0 0 4 3490638
05-jan-15 04:00 1 MARKETING 2909821187 0 0 0 0 0 4 3490638
05-jan-15 00:00 1 MARKETING 2909821187 0 0 0 0 0 4 3490638
05-jan-15 00:00 1 MARKETING 3363719406 0 602836 1 1228.4 602836 4 3490638
05-jan-15 00:00 1 MARKETING 701902966 0 0 0 0 0 4 3490638
04-jan-15 23:00 1 MARKETING 3363719406 0 348655 0 644.1 348655 2 2268225
04-jan-15 23:00 1 MARKETING 2909821187 0 0 0 0 0 2 2268225
04-jan-15 23:00 1 MARKETING 701902966 0 0 0 0 0 2 2268225
04-jan-15 21:00 1 MARKETING 3363719406 0 360663 1 296.8 360663 2 2268225
04-jan-15 21:00 1 MARKETING 701902966 0 0 0 0 0 2 2268225
04-jan-15 21:00 1 MARKETING 2909821187 0 0 0 0 0 2 2268225
04-jan-15 20:00 1 MARKETING 2909821187 0 0 0 0 0 2 2268225
04-jan-15 20:00 1 MARKETING 701902966 0 0 0 0 0 2 2268225
04-jan-15 20:00 1 MARKETING 3363719406 0 582629 0 3257.7 582629 2 2268225
04-jan-15 19:00 1 MARKETING 701902966 0 0 0 0 0 2 2268225
04-jan-15 19:00 1 MARKETING 2909821187 0 0 0 0 0 2 2268225
04-jan-15 19:00 1 MARKETING 3363719406 0 8362 0 227.5 8362 2 2268225
04-jan-15 16:00 1 MARKETING 3363719406 3 967916 1 723.1 967916 2 2268225
04-jan-15 16:00 1 MARKETING 2909821187 0 0 0 0 0 2 2268225
04-jan-15 16:00 1 MARKETING 701902966 0 0 0 0 0 2 2268225
Conclusion
From the output, we can understand that the sample query runs 2-4 times daily and takes 5-30 minutes on the average. Gets per execution goes up to a million. But the rows per execution is too small.
So if this is a batch kind of SQL, it may be okay. But it may be worth to investigate the scope of improving the query. While it is always a good idea to reduce the load as much as possible, the system wide impact of this query may be not very significant.
Alex Hayden
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