[gdal-dev] Re: [SoC] GDAL WKTRaster weekly report #9

Jorge Arévalo jorge.arevalo at gmail.com
Sun Aug 2 09:22:24 EDT 2009

Hello Even,

2009/8/2 Even Rouault <even.rouault at mines-paris.org>:
> Le Sunday 02 August 2009 13:56:52 Jorge Arévalo, vous avez écrit :
>> Hello,
>> So, I can create an array of children datasets to store the overviews,
>> but I shouldn't add the datasets as metatada under SUBDATASETS domain.
>> Is it right?
> Right, you shouldn't use SUBDATASETS for reporting overviews.

Thanks for confirmation

>> When I create an overview (a children dataset), its properties are the
>> same properties of the general dataset (his parent), apart from pixel
>> size, table name and column_name, of course. The problem appears when
>> sharing the PGconn object.
>> My code looks like this:
>> poDS->overview[i]->hPGconn = poDS->hPGconn
>> If each dataset's destructor (from the general one and from the
>> overviews) calls PQfinish over the same object, I get a segmentation
>> fault after the first time, of course. So, I have 2 options:
>> - Detect when a connection object has been freed, and perform only one
>> call. How should I do this?
>> - Don't share the PGconn object. Should I declare a static PGconn pointer?
>> What is the best approach?
> A static variable is rarely a good idea. It would prevent the user from having
> multiple WKTRaster datasets opened at the same time.
> Perhaps you should add a bBase flag to the WKTRasterDataset object (like it's
> done for the GTiffDataset). You set it to TRUE for the base dataset and to
> FALSE for the overviews. Then you only call PQfinish() if it is TRUE.

Very good idea. I think I'll use it. Many thanks!

> (I've no looked at what your code does recently, so the following remark might
> be irrelevant. But if you use transactions and their life cycle runs accross
> several calls of GDAL API, using the same connection object for the base
> dataset and its overviews might not be appropriate, or might require having a
> flag to know that a transaction is opened. --> You want to be able to mix
> RasterIO() calls on the base dataset and overviews)
No, I'm not using transactions just now, but I'll take it into account
if I use them. Thanks again.

Best regards,

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