Open a single db transaction for a single operation

[olsen232]

Description

Went through all the functions in the working copies and made sure they don't open more than one transaction in a sequence, or call more than one function in a sequence that would result in opening more than one transaction in a sequence.

This can mostly be easily fixed by opening a transaction that is reused, and that contains all the functions called. Any transactions that those functions open will reuse the one ongoing transaction.

A couple of things needed changing -

• we can't switch database engine mid-transaction to create the spatial index using gdal. Luckily, it seems like reusing the apsw engine works? Is there a downside?
• Rearranged some DELETEs so they didn't need PRAGMA defer_foreign_keys

[rcoup]

So I think we're effectively swapping from GDALs spatial index to Spatialite's here.

I want to dig in to figure out what that actually means in practice wrt differences.

[olsen232]

... can we use the GDAL engine everywhere??

No idea how this ties in with sqlalchemy as and when we get that working

[olsen232]

@rcoup is this something you are still looking into?

[rcoup]

AFAICS GDAL GPKG Spatial Index code and the Spatialite equivalent basically seem to do the same thing. Which I guess is because it's defined as an official GPKG extension.

👍