Performance issue in select when subselect is used

Question

We have a select statement that is using a where clause that has dates in. I've simplified the SQL below to demonstrate roughly what it does:
select * from user_activity
where starttime >= trunc(sysdate - 1) + 18/24
and starttime < trunc(sysdate) + 18/24
(this selects records from a table where a starttime field has values beween 6pm today and 6pm yesterday).
We are using this statement to create a materialized view which we refresh daily. Occasionally we have the need to refresh the data for a historic day, which means that we need to go in and change the where lines, e.g. to get data from 3 days ago instead of yesterday, the where clause becomes:
select * from user_activity
where starttime >= trunc(sysdate - 3) + 18/24
and starttime < trunc(sysdate - 2) + 18/24
Having to recreate the views like this is a nuisance, so we decided to be 'smart' and create a table with a setting in that we could use to control the number of days the view looks back. So if our table is called days_ago and field called days (with a single record, set to 1), we now have
select * from user_activity
where starttime >= trunc(sysdate - (select days from days_ago) + 18/24
and starttime < trunc(sysdate - ((select days from days_ago) + 1) + 18/24
The original SQL takes a few seconds to run. However the 'improved' version takes 25 minutes.
The days table only has 1 record in, selecting directly from it is instantaneous. Running the select on its own.
Does anything jump out as being daft in this approach? We cannot explain why the performance has suddenly dropped off for such a simple change.

Answer 1

Hi,
Do you really need a view to do this?
Can't you define a bind varibale, and use it in your query:
VARIABLE  days_ago   NUMBER
EXEC     :days_ago := 3;
SELECT      ...
WHERE     starttime >= TRUNC (SYSDATE - :days_ago') + (18 / 24)
AND     starttime <  TRUNC (SYSDATE - :days_ago') + (42 / 24)     -- 42 = 24 + 18If you really must have a view, then it might be faster if you got the parameter from a SYS_CONTEXT variable, rather than from a table.
Unfortunately, SYS_CONTEXT always returns a string, so you have to be carefule encoding the number as a string when you set the variable, and decoding it when you use the variable:
WHERE     starttime >= TRUNC (SYSDATE - TO_NUMBER ( SYS_CONTEXT ('MY_VIEW_NAMESPACE', 'DAYS_AGO'))) + (18 / 24)
AND     starttime <  TRUNC (SYSDATE - TO_NUMBER ( SYS_CONTEXT ('MY_VIEW_NAMESPACE', 'DAYS_AGO'))) + (42 / 24)     -- 42 = 24 + 18For more about SYS_CONTEXT, look it up in the SQL language m,anual, and follow the links there:
http://docs.oracle.com/cd/B28359_01/server.111/b28286/functions172.htm#sthref2268
If performance is important enough, consider storing the "fiscal day" as a separate (indexed) column. Starting in Oracle 11.1, you can use a virtual column for this. In earleir versions, you'd have to use a trigger. By "fiscal day", I mean:
TRUNC (starttime + (6/24))If starttime is between 18:00:00 on December 28 and 17:59:59 on December 29, this will return 00:00:00 on December 29. You could use it in a query (or view) like this:
WHERE     fiscal_day   = TRUNC (SYSDATE) - 2