Rails - Jul 2006 - Losing precision while copying interval type data (Postgres)

Hi

I am trying to use active record to copy some data.  One of the entry I 
am copying is type interval (Postgres); however, it lose some precision 
after I copied that column.

Here is what I did:
@newData.elapsedtime = @oldData.elapsedtime
@newData.save


Result:
both displayed as 00:00:02.453 in the table, however, if I use the 
following command to convert its value into float
( SELECT date_part(''epoch''::text, [table].elapsedtime) AS
date_part) AS
elapsedtime_sec

The original --> 2.4529983024597
After copy   --> 2.453

Is there any way to keep the original value?

Thank you very much!

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On Jul 26, 2006, at 5:06 , Victor Fan wrote:
> One of the entry I
> am copying is type interval (Postgres); however, it lose some  
> precision
> after I copied that column.
> The original --> 2.4529983024597
> After copy   --> 2.453
>
> Is there any way to keep the original value?
I''m not familiar with how Rails, the Postgres adapter, or your code  
handle the conversion between the Postgres interval type and the  
corresponding Ruby object, so I can''t say whether or not it''s
losing
precision there. My guess is that this is where the problem is as  
well, so what follows is just a bit of explanation of Postgres  
internals.

On the Postgres server side, it looks like the server was not  
compiled with the --enable-integer-datetimes flag. With --enable- 
integer-datetimes you''ll get microsecond precision. You''re
showing
much more that that, which means that datetimes (e.g., timestamps and  
intervals) are stored internally as floats. That precision you see  
there is not guaranteed and prone to all of the interesting aspects  
of floating point math.

If you need up to microsecond precision, see if you can''t get your  
server to be compiled with --enable-integer-datetimes. If you need  
precision greater than that, perhaps store number of seconds in a  
numeric column.

Don''t know if this helps, but perhaps it enlightens a bit. I''m
interested to see if someone pipes up about how a Postgres interval  
is translated to Ruby and back. (I suppose I could look a the source  
myself, but I''m not as of yet sufficiently motivated :) )

Michael Glaesemann
grzm seespotcode net

Michael Glaesemann wrote:
> 
> I''m not familiar with how Rails, the Postgres adapter, or your
code
> handle the conversion between the Postgres interval type and the
> corresponding Ruby object, so I can''t say whether or not
it''s losing
> precision there. My guess is that this is where the problem is as
> well, so what follows is just a bit of explanation of Postgres
> internals.
> 
> On the Postgres server side, it looks like the server was not
> compiled with the --enable-integer-datetimes flag. With --enable-
> integer-datetimes you''ll get microsecond precision.
You''re showing
> much more that that, which means that datetimes (e.g., timestamps and
> intervals) are stored internally as floats. That precision you see
> there is not guaranteed and prone to all of the interesting aspects
> of floating point math.
> 
> If you need up to microsecond precision, see if you can''t get your
> server to be compiled with --enable-integer-datetimes. If you need
> precision greater than that, perhaps store number of seconds in a
> numeric column.
> 
> Don''t know if this helps, but perhaps it enlightens a bit.
I''m
> interested to see if someone pipes up about how a Postgres interval
> is translated to Ruby and back. (I suppose I could look a the source
> myself, but I''m not as of yet sufficiently motivated :) )
> 
> Michael Glaesemann
> grzm seespotcode net
Thank you Michael for your reply

I did a bit of research, but couldn''t find anything useful so far.

Victor

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