RE: Slow query

A small follow up question:

Why will oracle do a to_date() on second query for each row returned, as the date mentioned is a literal and doesn't change for the duration of the query. Won't it be calculated once and used for all the rows with trunc() applied to each row?

Regards
Naveen

-----Original Message-----
Sent: Wednesday, March 05, 2003 3:04 PM
To: Multiple recipients of list ORACLE-L

Which version of Oracle ?

The second query has to do a trunc() and a to_date() on every single row. The first only has to do a to_char on every single row. There is a CPU difference between the two queries and if you run the execution plan through v9 with cpu_costing, you will find a difference in cost - though probably not enough to agree with your clock test.

A simple test on 8.1.7.4 and 9.2.0.2 where I created a table of 128,000 rows with a date column scattered inside a single date showed that one query used about twice the CPU of the other in v9, and four times in v8. (Unfortunately, in all cases, the CPU reported by Oracle was consistently larger than the elapsed time reported by SQL*Plus).

First check, though, is the test repeatable, or did something happen elsewhere to affect the elapsed time. (e.g. was one tested subject to a lot of delayed block cleanout).

NB - The execution plan shown is not the plan for either of the two statements, and I am fairly sure that neither statement could actually return 120,000 rows in 10 ms. Do your timings refer to a test where you changed the query to do a count() ?

Regards

Jonathan Lewis
http://www.jlcomp.demon.co.uk

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• Original Message ----- To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com> Sent: 05 March 2003 06:33

> Dear all,
>
> Please find below the 2 queries :
>
> select dep_date from test_date1
> where to_char(dep_date,'dd-mon-yyyy') = '12-jan-2002'
>
> select dep_date from test_date1
> where TRUNC(dep_date) = to_DATE( '12-jan-2002','dd-mon-yyyy')
>
> The execution plan for both the queries shows a FTS on test_date1.
>
> Execution Plan
> ----------------------------------------------------------
> 0 SELECT STATEMENT Optimizer=CHOOSE
> 1 0 SORT (AGGREGATE)
> 2 1 TABLE ACCESS (FULL) OF 'TEST_DATE1'
>
> The number of rows to be retrieved are 120010.
>
> The first query took 10msec to execute whereas the second took about
2
> secs.
>
> Could anyone help me figure out what the problem would be.
>
> TIA
>
> Best Regards
> Jai

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