Cannot create layer because "unable to open database file" in QGIS 3.2.0?

Question

I'm brand new to QGIS but 10+ years ago I was adept with ArcGIS. I installed 3.2.0 just a few days ago and all went well at first. I located/downloaded/imported the data I want and added a field to the table in each layer. I'm using Windows 10 Home v.1803, 64-bit. Now I want to create new layers using that new field. Screeching halt.

I can't create a new layer. I can't clip the layer. I can't filter the layer. I can't create anything else, apparently, because I keep getting some version of this error:

"Export to vector file failed. Error: Creation of data source failed (OGR error: sqlite3_open(NC GeoRegions.gpkg) failed: unable to open database file)"

I'm not sure what database it's trying to open, or I would check out its read-write permission. And anyway I can't believe the message is accurate. If QGIS cannot "open" a database, how in the world can it read that database to populate attribute tables? It makes no sense on its face.

What am I doing wrong?

Answer 1

The easy solution to your issue is to export using the shapefile format instead of the geopackage format.

See below if you want to know why you're having this issue.

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Geopackage is a database format. In one geopackage, you can have many layers. This is different from the shapefile format. Each shapefile layer is a separate file (actually 3-6 files, but you probably already know that).

You must have an existing geopackage before you can add a layer to it. When you try to export as a geopackage layer, QGIS can't find a geopackage to add the layer to. That's why you're getting the error message "unable to open database."

You must create a geopackage before you can add layers to it.

To create a geopackage database, go to the Layer menu > Create Layer > New Geopackage Layer. In the window that pops up, next to click the "..." button next to Database. Choose the folder where you want to put your database and type in a name. Click Save, then click OK.

Now you have a geopackage with one empty table. You can now export layers to this geopackage.

Answer 2

In case anyone else is also still encountering this error: I've come to realize that, quite often in PyQGIS, errors are actually caused by something that happens earlier. I had this exact problem when trying to call the v.split function, and in my case the issue was that for some reason, it defaulted to format="". Manually setting it to format="GPKG" via the 'GRASS_VECTOR_LCO' parameter solved the issue for me. (Naturally, only do this if you know the input will actually be a GeoPackage.)

Of course, this is a quite specific example. A more general tip for troubleshooting an issue like that, and how I figured this one out, was by taking a very close look at the logs, not just the actual error messages but also any easy-to-miss warnings in the grey text within.

Answer 3

Well... this isn't an answer exactly, but I found a workaround that seems to do the trick for now. I still wish I knew why doing what I'm supposed to do doesn't yield the result it's supposed to as far as almost any function that involves writing to some mystery database. BUT --

After trying every other tool, I found one that works. And in the end it's just as effective. Not elegant, not proper, not efficient. But effective.

One of the Data Management tools in the Vector menu is "Split Vector Layer." I've done this now several dozen times. Honestly I don't know what the difference is between "Split Vector Layer" and "Create New Shapefile Layer" because if I could do the latter, the result I'd expect is exactly what I get with the Split tool.

I've been wanting to create a subset of the primary layer, so I select the features I want and split them. The source layer remains intact, the new layer contains just the selected items. (The tool is more versatile than that, but that's what I need it do do.) Imagine starting with a State layer but you want a separate layer for each county. This can do that, and each new layer contains all the fields of the parent layer.

In order to avoid confusion because of the default naming convention for the new layer, I go to the output folder from Windows Explorer and rename the new shapefile and its associated files.

Answer 4

I recently produced this exact same error when trying to create an gpkg output in a folder that doesn't exist yet. I added a routine that first creates the folder and that solved the issue.

# create output folder if it doesn't exist
if not os.path.exists(os.path.dirname(dataset_output)):
  os.makedirs(os.path.dirname(dataset_output))