Question

I have to benchmark a query – currently I need to know how adding parameter to select result set(FIELD_DATE1) will affect sql execution time. There is administration restrictions in db so I can not use debug. So I wrote a query:

SELECT COUNT(*), MIN(XXXT), MAX(XXXT) 
  FROM ( select distinct ID AS XXXID, sys_extract_utc(systimestamp) AS XXXT
              , FIELD_DATE1 AS XXXUT  
           from XXXTABLE 
          where FIELD_DATE1 > '20-AUG-06 02.23.40.010000000 PM' );

Will output of query show real times of query execution

Answer 1

It’s possible in this query that SYSTIMESTAMP would be evaluated once, and the same value associated with every row, or that it would be evaluated once for each row, or something in-between. It also possible that all the rows would be fetched from table, then SYSTIMESTAMP evaluated for each one, so you wouldn’t be getting an accurate account of the time taken by the whole query. Generally, you can’t rely on order of evaluation within SQL, or assume that a function will be evaluated once for each row where it appears.

Generally the way I would measure execution time would be to use the client tool to report on it. If you’re executing the query in SQLPlus, you can SET TIMING ON to have it report the execution time for every statement. Other interactive tools probably have similar features.