Filtering SQL query by row and by date range

Question

I have a time indexed Oracle DB which I'm trying to query by date range. I also want to do data reduction in the query so I don't get overwhelmed with too much data.

The stand alone date query (2352 rows in 0.203s):

select oracle_time from t_ssv_soh_packets0
where oracle_time >= TIMESTAMP '2009-01-01 00:00:00' 
AND oracle_time <= TIMESTAMP '2009-01-31 00:00:00' 

The stand along reduction query (1017 in 0.89s):

select oracle_time from t_ssv_soh_packets0
where (rowid,0) in (select rowid, mod(rownum,50) from t_ssv_soh_packets0)

When I try to combine them it takes forever (48 rows in 32.547s):

select oracle_time from t_ssv_soh_packets0
where oracle_time >= TIMESTAMP '2009-01-01 00:00:00' 
AND oracle_time <= TIMESTAMP '2009-01-31 00:00:00' 
AND (rowid,0) in (select rowid, mod(rownum,50) from t_ssv_soh_packets0)

Obviously I'm doing something fundamentally wrong here but I don't know how else to both query by date and reduce the data.

Answer 1

Thanks to both 'Narveson' and 'nate c' for the pointers I finally figured it out. Here is the (probably Oracle specific) query that I came up with:

select oracle_time from t_ssv_soh_packets0 where oracle_time >= TIMESTAMP '2009-01-01 00:00:00' AND oracle_time <= TIMESTAMP '2009-01-31 00:00:00' group by oracle_time, rownum having mod(rownum, 50)=0

This query returns 47 rows in 0.031s. The original time query had 2352 rows so that makes sense.

The ORAFAQ helped me get to the final solution.

Answer 2

You are evaluating your reduction logic against rows that are not in your chosen date range.

Apply the reduction logic to a subquery containing your date range.

LATER: Here's what I meant.

select oracle_time from (
  select oracle_time, rownum as limited_row_num
  from t_ssv_soh_packets0 
  where oracle_time >= TIMESTAMP '2009-01-01 00:00:00'  
  AND oracle_time <= TIMESTAMP '2009-01-31 00:00:00'  
) as time_range
where mod(limited_row_num,50) =  0

Answer 3

Get rid of the in

Why use this?:

select oracle_time from t_ssv_soh_packets0
where (rowid,0) in (select rowid, mod(rownum,50) from t_ssv_soh_packets0)

Your only condition is mod(rownum, 50)

select * from t where mod(rownum, 50)=0

last line should be AND mod(rownnum,50)=0 not a self join with an in.

Answer 4

You can also let Oracle choose a random sample from the result by applying the SAMPLE() clause:

SELECT oracle_time 
FROM t_ssv_soh_packets0
WHERE ...
SAMPLE(50)

Will return randomly 50% percent of the rows