[gdal-dev] Decoding feature blobs and tracking FIDs in MVT datasets

[Linda Kladivová]

Hi,




I’d like to ask about one specific aspect of the MVT driver. In my workflow,
I need to track the FIDs of features inside an MVT dataset together with 
their corresponding internal codes (actual parcel IDs).




My use case involves an MVT dataset of about 20 million features that is 
frequently updated. For this purpose, I have implemented an extension of the
MVT driver that can open an MVT dataset in update mode and create/delete 
features based on changes detected by a preceding ETL process. Updates 
typically affect only a few individual features every hour.




After the initial loading of the full dataset using ogr2ogr I need a way to 
iterate through the dataset and build a separate table mapping each internal
FID to the parcel’s real ID. I have tried opening and inspecting the temp.db
file (which I do not delete):




GDALDataset* poTempDb =
    (GDALDataset*) GDALOpenEx(dstTempDb.c_str(),
                              GDAL_OF_VECTOR, nullptr, nullptr, nullptr);

OGRLayer* poTempLayer = poTempDb->GetLayerByName("temp");
poTempLayer->ResetReading();

OGRFeature* poFeat = nullptr;

while ((poFeat = poTempLayer->GetNextFeature()) != nullptr)
{
    GIntBig fid = poFeat->GetFID();
    std::cout << "FID=" << fid << " — ";
    int idx = poFeat->GetFieldIndex("feature");
    int blobSize = 0;
    const GByte* pBlob = poFeat->GetFieldAsBinary(idx, &blobSize);
    std::cout << " Blob size=" << blobSize << std::endl;
}



I can successfully access the feature blob, but I need to decode it in order
to extract my custom parcel ID attribute and store those values externally. 
This would allow me to keep track of which parcel corresponds to which 
internal FID.


This is a one-time (potentially slow) process. Afterward, during incremental
updates, I will be calling CreateFeature and DeleteFeature (and updating my 
external mapping table accordingly). For edits, I currently use a 
combination of DeleteFeature and CreateFeature (I haven’t implemented 
SetFeature yet).




My question is:
Is there a way to decode this blob using existing GDAL/MVT functionality, or
would this require implementing a new function?

Thank you very much for your help.




Linda Karlovská
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