RE: cheapest way to access every 15th row in table ordered by timestamp column
- From: "Stephens, Chris" <Chris.Stephens@xxxxxxx>
- To: Michael Moore <michaeljmoore@xxxxxxxxx>
- Date: Wed, 19 Jan 2011 12:20:24 -0600
Yep, that's what we went with.
Thanks for every ones help.
From: Michael Moore [mailto:michaeljmoore@xxxxxxxxx]
Sent: Wednesday, January 19, 2011 12:17 PM
To: Stephens, Chris
Cc: ORACLE-L
Subject: Re: cheapest way to access every 15th row in table ordered by
timestamp column
These give different results:
SELECT *
FROM (SELECT object_name,
ROW_NUMBER () OVER (ORDER BY timestamp) rnum,
timestamp
FROM dba_objects)
WHERE MOD (rnum, 15) = 0;
SELECT *
FROM ( SELECT object_name, ROWNUM rnum, timestamp
FROM dba_objects
ORDER BY timestamp)
WHERE MOD (rnum, 15) = 0
ORDER BY rnum;
Different because in the 1st select the rows are sorted first, then the
row_number assigned.
In the 2nd example, the row numbers are assigned and then the rows are sorted.
I think the first format is the one you want.
Mike
On Wed, Jan 19, 2011 at 8:20 AM, Stephens, Chris
<Chris.Stephens@xxxxxxx<mailto:Chris.Stephens@xxxxxxx>> wrote:
11.2.0.2 on Linux 5
We have a lab instrument recording information every 2 seconds into a table.
The scientists pull that data into excel for analysis (I hope to look at this
excel spreadsheet in the very near future now that I've read chapter 9 of Pro
Oracle SQL).
They are requesting a view that picks out every 15th row of data going back 30
days.
Can anyone think of a more efficient way to do it than this:
SELECT x.col,
x.t_stamp
FROM ( SELECT col,
t_stamp
rownum rn
FROM t
WHERE t_stamp >= TRUNC( SYSDATE - 30 )) x
WHERE MOD( x.rn, 15 ) = 0;
It feels like there should be a better way to do it.
chris
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