Обсуждение: returning row numbers in select
Is there a way to return an integer row number for a query? Note that there may be a large number of rows so I would rather not have joined selects... For a rather simple query: SELECT timestamp from test WHERE timestamp > '2004-02-02 04:15:00.00 +0' AND timestamp < '2004-02-02 04:15:10.00 +0'; where 'test' is Column | Type | Modifiers -----------+--------------------------+----------- timestamp | timestamp with time zone | value | double precision | Indexes: table_timestamp I to return a sequential row number beginning at 1? i.e. row| timestamp ---+---------------------------- 1 2004-02-01 23:15:00.824-05 2 2004-02-01 23:15:01.824-05 3 2004-02-01 23:15:02.824-05 ... My reason for wanting row numbers is so I can use a 'MOD(row_number, n)' to get the nth row from the table. Cheers, Randall
> I to return a sequential row number beginning at 1? > > i.e. > > row| timestamp > ---+---------------------------- > 1 2004-02-01 23:15:00.824-05 > 2 2004-02-01 23:15:01.824-05 > 3 2004-02-01 23:15:02.824-05 > ... > > My reason for wanting row numbers is so I can use a 'MOD(row_number, > n)' to get the nth row from the table. Doesn't LIMIT and OFFSET do the job? http://www.postgresql.org/docs/7.3/interactive/queries-limit.html Bye, Chris.
>> I to return a sequential row number beginning at 1? >> >> i.e. >> >> row| timestamp >> ---+---------------------------- >> 1 2004-02-01 23:15:00.824-05 >> 2 2004-02-01 23:15:01.824-05 >> 3 2004-02-01 23:15:02.824-05 >> ... >> >> My reason for wanting row numbers is so I can use a 'MOD(row_number, >> n)' to get the nth row from the table. Correction, I don't want to simply get the nth row, I want all rows that are divisible by n. Essentially, the timestamp is at a regular interval and I want a way of selecting rows at different sampling intervals. > Doesn't LIMIT and OFFSET do the job? > http://www.postgresql.org/docs/7.3/interactive/queries-limit.html It would if I only wanted an offset butI want a query to return the first, fifth, and tenth, and so on row. This would be 'MOD(row_num, 5)' but given that I don't know the number of rows a priori, it is difficult to write a LIMIT. Moreover, the offset doesn't make it easy to get the first row. Unless, of course, I am missing something obvious? Cheers, Randall
Randall Skelton <skelton@brutus.uwaterloo.ca> writes:
> Correction, I don't want to simply get the nth row, I want all rows
> that are divisible by n. Essentially, the timestamp is at a regular
> interval and I want a way of selecting rows at different sampling
> intervals.
Couldn't you code this as a WHERE test on the timestamp?
regards, tom lane
Maybe by using a sequence and a function. The sequece to generate the row number. The a function 1) to reset the sequence and 2) to perform a select with the first column nextval(seq) and the column the timestamp However, I am a newbie with PostgreSQL and I am not sure it this will work correctly...you might have to play with it (or wait for somebody with more experience than me). Bernard On Wednesday 10 March 2004 16:23, Randall Skelton wrote: > >> I to return a sequential row number beginning at 1? > >> > >> i.e. > >> > >> row| timestamp > >> ---+---------------------------- > >> 1 2004-02-01 23:15:00.824-05 > >> 2 2004-02-01 23:15:01.824-05 > >> 3 2004-02-01 23:15:02.824-05 > >> ... > >> > >> My reason for wanting row numbers is so I can use a 'MOD(row_number, > >> n)' to get the nth row from the table. > > Correction, I don't want to simply get the nth row, I want all rows > that are divisible by n. Essentially, the timestamp is at a regular > interval and I want a way of selecting rows at different sampling > intervals. > > > Doesn't LIMIT and OFFSET do the job? > > http://www.postgresql.org/docs/7.3/interactive/queries-limit.html > > It would if I only wanted an offset butI want a query to return the > first, fifth, and tenth, and so on row. This would be 'MOD(row_num, > 5)' but given that I don't know the number of rows a priori, it is > difficult to write a LIMIT. Moreover, the offset doesn't make it easy > to get the first row. Unless, of course, I am missing something > obvious? > > Cheers, > Randall > > > ---------------------------(end of broadcast)--------------------------- > TIP 2: you can get off all lists at once with the unregister command > (send "unregister YourEmailAddressHere" to majordomo@postgresql.org)
>> Correction, I don't want to simply get the nth row, I want all rows
>> that are divisible by n. Essentially, the timestamp is at a regular
>> interval and I want a way of selecting rows at different sampling
>> intervals.
>
> Couldn't you code this as a WHERE test on the timestamp?
That would be ideal as it is theoretically possible for there to be
missing rows due to sampling errors; nevertheless, a WHERE test doesn't
seem obvious to me. Can you please post an example? The time spacing
between rows is 1 second but I want my select statement to return rows
every 5 seconds (see marked lines below). I've tried various interval
operations but I don't really understand how to relate the timestamp
and and the interval.
SELECT timestamp FROM test WHERE timestamp > '2004-02-02 04:15:00.00
+0' AND timestamp < '2004-02-02 04:15:10.00 +0' ORDER BY timestamp;
timestamp
----------------------------
2004-02-01 23:15:00.824-05 *
2004-02-01 23:15:01.824-05
2004-02-01 23:15:02.824-05
2004-02-01 23:15:03.824-05
2004-02-01 23:15:04.824-05
2004-02-01 23:15:05.824-05 *
2004-02-01 23:15:06.824-05
2004-02-01 23:15:07.824-05
2004-02-01 23:15:08.824-05
2004-02-01 23:15:09.824-05
2004-02-01 23:15:10.824-05 *
...
Thanks,
Randall
Randall Skelton <skelton@brutus.uwaterloo.ca> writes:
>> Couldn't you code this as a WHERE test on the timestamp?
> That would be ideal as it is theoretically possible for there to be
> missing rows due to sampling errors; nevertheless, a WHERE test doesn't
> seem obvious to me. Can you please post an example?
Something like
WHERE (EXTRACT(EPOCH FROM timestamp)::numeric % 5) = 0;
The EXTRACT function returns double precision, but there's no double
modulo operator for some reason, hence the cast to numeric which does
have one.
regards, tom lane
On Wed, Mar 10, 2004 at 17:25:49 -0500, Randall Skelton <skelton@brutus.uwaterloo.ca> wrote: > > That would be ideal as it is theoretically possible for there to be > missing rows due to sampling errors; nevertheless, a WHERE test doesn't > seem obvious to me. Can you please post an example? The time spacing > between rows is 1 second but I want my select statement to return rows > every 5 seconds (see marked lines below). I've tried various interval > operations but I don't really understand how to relate the timestamp > and and the interval. You could extract seconds from timestamp, cast to integer and apply the mod function and test against whichever remainder you want.
On Wednesday 10 March 2004 12:25 pm, Randall Skelton wrote:
> Is there a way to return an integer row number for a query? Note
> that there may be a large number of rows so I would rather not have
> joined selects...
<snip>
Well...if your result has a unique column you can do something like
this:
steve=# select (select count(*) from bar as barcount where
barcount.sec<=bar.sec) as rownum, sec from bar order by sec;
rownum | sec
--------+------------
1 | 1063966688
2 | 1063966689
3 | 1063966690
4 | 1063966691
5 | 1063966692
6 | 1063966693
7 | 1063966694
8 | 1063966695
9 | 1063966696
10 | 1063966697
11 | 1063966698
12 | 1063966699
13 | 1063966700
14 | 1063966701
15 | 1063966702
16 | 1063966703
17 | 1063966704
18 | 1063966705
As you might guess, this is not a fast query - more of a brute-force
kludge. It's likely that you will be better off postprocessing the
query to select every n records or possibly writing a function that
will handle the situation.
Cheers,
Steve