Re: very slow when writing query to file

or maybe the retrieved data in RAM are somehow lazy bound ... ?

On 11/02/2011 08:58 AM, boris pezzatti wrote:
> Thank you Fernando for reproducing this.
> I suspect there must be some part of code in the
> * for each row
> * for each column
> loops that result inefficient only on some machines or OS's (I'm using
> Archlinux).
> In fact the extra time I and Fernando get can not only be attributed
> to adding commas, " and line feeds. The same file which I retrived in
> more than one hour (32Mb), could be saved with openoffice in 10
> seconds, changing column separators, text delimiters and encoding.
>
> Could it be possible that the use of e.g. error catching in the loops
> results in poor performance on some systems? (I'm not a c++ programmer
> ...)
>
>
>
>
>
>
>
> On 11/01/2011 11:23 AM, Guillaume Lelarge wrote:
>> On Mon, 2011-10-31 at 18:26 -0300, Fernando Hevia wrote:
>>> [...]
>>> I could reproduce the issue in a fresh Windows 7 install with no other apps
>>> running other than pgAdmin v1.14.0.
>>> > From what I could see, the execute-to-file function runs in 2 stages:
>>> 1. Rows are retrieved from DB server to RAM
>>> 2. Rows are written from RAM to file
>>>
>> That's right.
>>
>>> The delay clearly occurs in step 2.
>> That's also right.
>>
>>> While with small datasets (<2000) the writing to disk delay is barely
>>> perceivable, when the number of rows is incremented (>10k) it is quite
>>> distinctive how step 1 keeps completing in the expected time frame but step
>>> 2 takes much much longer. In any case it should be the other way around.
>>>
>> Nope, step 2 does a lot of work.
>>
>>> With 10k rows (2.3 MB) it took 3.5 seconds to retrieve data from DB and 40
>>> seconds to write the file to an SATA 7200 disk with write-through cache.
>>> With 100k rows (23 MB) the DB retrieve went for 35 seconds while the file
>>> writing part took over 4 minutes (didn't wait for it to finish).
>>>
>> I don't have the same numbers:
>>
>> * 1000 : 1s
>> * 10000 : 1s
>> * 200000 : 7s
>> * 1000000 : 18s
>>
>>> The file is being written at an avg 60 KB per second, which is extremely
>>> slow.
>> If the only thing pgAdmin does was writing, I would agree. But,
>> actually, it does a lot more things:
>>
>> * for each row
>> * for each column
>> * adds the column separator, if needed
>> * grabs one cell's value
>> * quotes the value, if needed (which also means doubling the quote
>> if it's within the value)
>> * adds the line separator
>> * converts it to the encoding, if needed
>> * writes it to the file
>>
>> That could take some time.
>>
>> I searched if there were some parts that took much longer than others,
>> but failed to find one.
>>
>>
>
> --
> *Boris Pezzatti
> Swiss Federal Research Institute WSL
> *
> Research unit Community Ecology
> Team Insubric Ecosystems
> via Belsoggiorno 22
> CH-6500 Bellinzona
> Switzerland phone direct ++41 91 821 52 32
> phone ++41 91 821 52 30
> fax ++41 91 821 52 39
> boris(dot)pezzatti(at)wsl(dot)ch
> http://www.wsl.ch <http://www.wsl.ch/>
>
>

--
*Boris Pezzatti
Swiss Federal Research Institute WSL
*
Research unit Community Ecology
Team Insubric Ecosystems
via Belsoggiorno 22
CH-6500 Bellinzona
Switzerland phone direct ++41 91 821 52 32
phone ++41 91 821 52 30
fax ++41 91 821 52 39
boris(dot)pezzatti(at)wsl(dot)ch
http://www.wsl.ch <http://www.wsl.ch/>