Oracle 8i date function slow - performance

Question

I'm trying to run the following PL/SQL on an Oracle 8i server (old, I know):
select
-- stuff --
from
s_doc_quote d,
s_quote_item i,
s_contact c,
s_addr_per a,
cx_meter_info m
where
d.row_id = i.sd_id
and d.con_per_id = c.row_id
and i.ship_per_addr_id = a.row_id(+)
and i.x_meter_info_id = m.row_id(+)
and d.x_move_type in ('Move In','Move Out','Move Out / Move In')
and i.prod_id in ('1-QH6','1-QH8')
and d.created between add_months(trunc(sysdate,'MM'), -1) and sysdate
;
Execution is incredibly slow however. Because the server is taken down around midnight each night, it often fails to complete in time.
The execution plan is as follows:
SELECT STATEMENT 1179377
NESTED LOOPS 1179377
NESTED LOOPS OUTER 959695
NESTED LOOPS OUTER 740014
NESTED LOOPS 520332
INLIST ITERATOR
TABLE ACCESS BY INDEX ROWID S_QUOTE_ITEM 157132
INDEX RANGE SCAN S_QUOTE_ITEM_IDX8 8917
TABLE ACCESS BY INDEX ROWID S_DOC_QUOTE 1
INDEX UNIQUE SCAN S_DOC_QUOTE_P1 1
TABLE ACCESS BY INDEX ROWID S_ADDR_PER 1
INDEX UNIQUE SCAN S_ADDR_PER_P1 1
TABLE ACCESS BY INDEX ROWID CX_METER_INFO 1
INDEX UNIQUE SCAN CX_METER_INFO_P1 1
TABLE ACCESS BY INDEX ROWID S_CONTACT 1
INDEX UNIQUE SCAN S_CONTACT_P1 1
If I change the following where clause however:
and d.created between add_months(trunc(sysdate,'MM'), -1) and sysdate
To a static value, such as:
and d.created between to_date('20110101','yyyymmdd') and sysdate
the execution plan becomes:
SELECT STATEMENT 5
NESTED LOOPS 5
NESTED LOOPS OUTER 4
NESTED LOOPS OUTER 3
NESTED LOOPS 2
TABLE ACCESS BY INDEX ROWID S_DOC_QUOTE 1
INDEX RANGE SCAN S_DOC_QUOTE_IDX1 3
INLIST ITERATOR
TABLE ACCESS BY INDEX ROWID S_QUOTE_ITEM 1
INDEX RANGE SCAN S_QUOTE_ITEM_IDX4 4
TABLE ACCESS BY INDEX ROWID S_ADDR_PER 1
INDEX UNIQUE SCAN S_ADDR_PER_P1 1
TABLE ACCESS BY INDEX ROWID CX_METER_INFO 1
INDEX UNIQUE SCAN CX_METER_INFO_P1 1
TABLE ACCESS BY INDEX ROWID S_CONTACT 1
INDEX UNIQUE SCAN S_CONTACT_P1 1
which begins to return rows almost instantly.
So far, I've tried replacing the dynamic date condition with bind variables, as well as using a subquery which selects a dynamic date from the dual table. Neither of these methods have helped improve performance so far.
Because I'm relatively new to PL/SQL, I'm unable to understand the reasons for such substantial differences in the execution plans.
I'm also trying to run the query as a pass-through from SAS, but for the purposes of testing the execution speed I've been using SQL*Plus.
EDIT:
For clarification, I've already tried using bind variables as follows:
var start_date varchar2(8);
exec :start_date := to_char(add_months(trunc(sysdate,'MM'), -1),'yyyymmdd')
With the following where clause:
and d.created between to_date(:start_date,'yyyymmdd') and sysdate
which returns an execution cost of 1179377.
I would also like to avoid bind variables if possible as I don't believe I can reference them from a SAS pass-through query (although I may be wrong).

Answer 1

I doubt that the problem here has much to do with the execution time of the ADD_MONTHS function. You've already shown that there is a significant difference in the execution plan when you use a hardcoded minimum date. Big changes in execution plans generally have much more impact on run time than function call overhead is likely to, although potentially different execution plans can mean that the function is called many more times. Either way the root problem to look at is why you aren't getting the execution plan you want.
The good execution plan starts off with a range scan on S_DOC_QUOTE_IDX1. Given the nature of the change to the query, I assume this is an index on the CREATED column. Often the optimizer will not choose to use an index on a date column when the filter condition is based on SYSDATE. Because it is not evaluated until execution time, after the execution plan has been determined, the parser cannot make a good estimate of the selectivity of the date filter condition. When you use a hardcoded start date instead, the parser can use that information to determine selectivity, and makes a better choice about the use of the index.
I would have suggested bind variables as well, but I think because you are on 8i the optimizer can't peek at bind values, so this leaves it just as much in the dark as before. On a later Oracle version I would expect that the bind solution would be effective.
However, this is a good case where using literal substitution is probably more appropriate than using a bind variable, since (a) the start date value is not user-specified, and (b) it will remain constant for the whole month, so you won't be parsing lots of slightly different queries.
So my suggestion is to write some code to determine a static value for the start date and concatenate it directly into the query string before parsing & execution.

Answer 2

First of all, the reason you are getting different execution time is not because Oracle executes the date function a lot. The execution of this SQL function, even if it is done for each and every row (it probably is not by the way), only takes a negligible amount of time compared to the time it takes to actually retrieve the rows from disk/memory.
You are getting completely different execution times because, as you have noticed, Oracle chooses a different access path. Choosing one access path over another can lead to orders of magnitudes of difference in execution time. The real question therefore, is not "why does add_months takes time?" but:
Why does Oracle choose this particular unefficient path while there is a more efficient one?
To answer this question, one must understand how the optimizer works. The optimizer chooses a particular access path by estimating the cost of several access paths (all of them if there are only a few tables) and choosing the execution plan that is expected to be the most efficient. The algorithm to determine the cost of an execution plan has rules and it makes its estimation based on statistics gathered from your data.
As all estimation algorithms, it makes assumptions about your data, such as the general distribution based on min/max value of columns, cardinality, and the physical distribution of the values in the segment (clustering factor).
How this applies to your particular query
In your case, the optimizer has to make an estimation about the selectivity of the different filter clauses. In the first query the filter is between two variables (add_months(trunc(sysdate,'MM'), -1) and sysdate) while in the other case the filter is between a constant and a variable.
They look the same to you because you have substituted the variable by its value, but to the optimizer the cases are very different: the optimizer (at least in 8i) only computes an execution plan once for a particular query. Once the access path has been determined, all further execution will get the same execution plan. It can not, therefore, replace a variable by its value because the value may change in the future, and the access plan must work for all possible values.
Since the second query uses variables, the optimizer cannot determine precisely the selectivity of the first query, so the optimizer makes a guess, and that results in your case in a bad plan.
What can you do when the optimizer doesn't choose the correct plan
As mentionned above, the optimizer sometimes makes bad guesses, which result in suboptimal access path. Even if it happens rarely, this can be disastrous (hours instead of seconds). Here are some actions you could try:
Make sure your stats are up-to-date. The last_analyzed column on ALL_TABLES and ALL_INDEXES will tell you when was the last time the stats were collected on these objects. Good reliable stats lead to more accurate guesses, leading (hopefully) to better execution plan.
Learn about the different options to collect statistics (dbms_stats package)
Rewrite your query to make use of constants, when it makes sense, so that the optimizer will make more reliable guesses.
Sometimes two logically identical queries will result in different execution plans, because the optimizer will not compute the same access paths (of all possible paths).
There are some tricks you can use to force the optimizer to perform some join before others, for example:
Use rownum to materialize a subquery (it may take more temporary space, but will allow you to force the optimizer through a specific step).
Use hints, although most of the time I would only turn to hints when all else fails. In particular, I sometimes use the LEADING hint to force the optimizer to start with a specific table (or couple of table).
Last of all, you will probably find that the more recent releases have a generally more reliable optimizer. 8i is 12+ years old and it may be time for an upgrade :)
This is really an interesting topic. The oracle optimizer is ever-changing (between releases) it improves over time, even if new quirks are sometimes introduced as defects get corrected. If you want to learn more, I would suggest Jonathan Lewis' Cost Based Oracle: Fundamentals

Answer 3

That's because the function is run for every comparison.
sometimes it's faster to put it in a select from dual:
and d.created
between (select add_months(trunc(sysdate,'MM'), -1) from dual)
and sysdate
otherwise, you could also join the date like this:
select
-- stuff --
from
s_doc_quote d,
s_quote_item i,
s_contact c,
s_addr_per a,
cx_meter_info m,
(select add_months(trunc(sysdate,'MM'), -1) as startdate from dual) sd
where
d.row_id = i.sd_id
and d.con_per_id = c.row_id
and i.ship_per_addr_id = a.row_id(+)
and i.x_meter_info_id = m.row_id(+)
and d.x_move_type in ('Move In','Move Out','Move Out / Move In')
and i.prod_id in ('1-QH6','1-QH8')
and d.created between sd.startdate and sysdate
Last option and actually the best chance of improved performance: Add a date parameter to the query like this:
and d.created between :startdate and sysdate
[edit]
I'm sorry, I see you already tried options like these. Still odd. If the constant value works, the bind parameter should work as well, as long as you keep the add_months function outside the query.

Answer 4

This is SQL. You may want to use PL/SQL and save the calculation add_months(trunc(sysdate,'MM'), -1) into a variable first ,then bind that.
Also, I've seen SAS calcs take a long while due to pulling data across the network and doing additional work on each row it processes. Depending on your environment, you may consider creating a temp table to store the results of these joins first, then hitting the temp table (try a CTAS).