The speed of 'extract(year from sysdate)' - sql

Question

I have two queries:
with tmp as (
select asy.aim_student_id, ast.aim_test, asq.response
from aim_student_test ast
join aim_student_qst asq on (asq.aps_yr = ast.aps_yr and asq.aim_test = ast.aim_test and asq.aim_id = ast.aim_id)
join aim_student_yr asy on (asy.aps_yr = ast.aps_yr and asy.aim_student_yr_id = ast.aim_student_yr_id)
where asq.aps_yr = '2012'
and asq.qst_num = 1)
select aim_student_id, aim_test, response
from tmp
where response is null
-- execution-time: 0.032 seconds
define this_year = extract(year from sysdate)
with tmp as (
select asy.aim_student_id, ast.aim_test, asq.response
from aim_student_test ast
join aim_student_qst asq on (asq.aps_yr = ast.aps_yr and asq.aim_test = ast.aim_test and asq.aim_id = ast.aim_id)
join aim_student_yr asy on (asy.aps_yr = ast.aps_yr and asy.aim_student_yr_id = ast.aim_student_yr_id)
where asq.aps_yr = &this_year
and asq.qst_num = 1)
select aim_student_id, aim_test, response
from tmp
where response is null
-- execution-time: 82.202 seconds
The only difference is that in one I use '2012' and the other I implement extract(year from sysdate).
I can only imagine that Oracle is computing extract(year from sysdate) for EVERY record it checks, and that I just can't figure out how to make it compute this once and use it as a variable. Searching has not returned me the answers I seek... so I come to the magicians of SO.com. HOW do I properly use
extract(year from sysdate)
as a variable?

Answer 1

Using &this_year in the query causes a substitution of the string extract(year from sysdate), so the second query actually has:
where asq.aps_yr = extract(year from sysdate)
which you can see from the second explain plan. That in itself probably isn't the problem; what's possibly slowing it down is that doing this is changing the plan from an index range scan to an index skip scan against aim_student_qstp1. The real difference is that in the fast version you're comparing asq.aps_yr to a string ('2012'), in the second it's a number (2012), and - as also shown in the explain plan - this is causing it to do to_number(asq.aps_yr) which is stopping the index being used as you expect.
You could fix this in your code by making it:
where asq.aps_yr = to_char(&this_year)
If you want to calculate it once before the query runs and then use it as a variable, there are at least two ways (in SQL*Plus/SQL Developer). Sticking with substitution variables you can use the column commands instead of the define:
column tmp_this_year new_value this_year
set termout off
select extract(year from sysdate) as tmp_this_year from dual;
set termout on
set verify off
... which makes &this_year=2012 (the termout changes just make the actual retrieval invisible, and the verify stops it telling you when it uses the substitution; both so you don't get extra output in your script), and change your query to have:
where asq.aps_yr = '&this_year'
... so the value is treated as a string, making a to_char() unnecessary.
Or you can use bind variables:
var this_year varchar2(4);
set feedback off;
exec :this_year := extract(year from sysdate);
... and then your query has:
where asq.aps_yr = :this_year
Note that in this case you don't need the quotes because the bind variable is defined as a string already - there's an implicit conversion in the exec that sets it.

Answer 2

I doubt the difference is due to extracting the year from the date. I'm pretty sure Oracle would only be extracting the year once, since it is using a variable in the second case.
The difference is due to the execution path used by the query. You would need to post the execution plans to really see the difference. Using an explicit constant gives the optimizer more information for choosing an optimal query plan.
For instance, if the data is partitioned by year, then with a constant year, Oracle can determine which partition has the data. In the second case, Oracle might not recognize the value as a constant, and require reading all data partitions. This is just an example of what might happen -- I'm not sure what Oracle does in this case.