Re: Best way to construct PostgreSQL ArrayType (_int4) from C int array

2011/4/27 Adrian Schreyer <ams214(at)cam(dot)ac(dot)uk>:
> The largest arrays I expect at the moment are more or less sparse
> vectors of around 4.8k elements and I have noticed that the
> input/output (C/C++ extension) does not scale well with the number of
> elements in the array.
>
> Using a function that sums all elements in the array, this is the time
> it takes for ~150k arrays of various sizes (including ordering desc
> and limit 10):
>

PostgreSQL doesn't use a index for access to array fields. So access
to fields of packed arrays can be slower for higher subscripts.

Regards

Pavel Stehule

> 128: 61ms
> 256: 80ms
> 512: 681ms
> 1024 1065ms
> 2048 7682ms
> 4096 21332ms
>
> That's why I thought that the construction of the PostgreSQL array was
> not optimal.
>
> On Wed, Apr 27, 2011 at 14:49, Merlin Moncure <mmoncure(at)gmail(dot)com> wrote:
>> On Wed, Apr 27, 2011 at 6:02 AM, Adrian Schreyer <ams214(at)cam(dot)ac(dot)uk> wrote:
>>> At the moment I am using the following code to construct a PostgreSQL
>>> array from a C array in my C extension but I am not so sure if this is
>>> really the best solution:
>>>
>>> const int *data = array.data(); // C array
>>> Datum *d = (Datum *) palloc(sizeof(Datum) * size);
>>>
>>> for (int i = 0; i < size; i++) d[i] = Int32GetDatum(data[i]);
>>>
>>> ArrayType *a = construct_array(d, size, INT4OID, sizeof(int4), true, 'i');
>>>
>>> Is this okay or is there a better solution (existing function in the
>>> PostgreSQL source for example)?
>>
>> That's pretty much the best way AFAIK. Int32GetDatum doesn't do
>> anything fancy -- it's just a 32 bit mask/assignment.  constructing
>> the array at once is going to be a lot better than incrementally
>> creating it.  Do you expect the arrays to be large, say bigger than
>> 10k elements?
>>
>> merlin
>>
>
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