This is mainly useful for Python, this way we can probably
avoid calling fields() and all related cost when all we need
is colum names for display.
I'm not very happy with the method name and I would probably just
drop the old signature for executeSql and replace it with the new
one, unfortunately is public API.
But if everybody is happy we could consider an API break.
Fixes#38092 by adding an optional QgsFeedback argument to
the executeSql method and by implementing the PQCancel
method in the PG provider internals.
While the cancellation works well for all supported provider while
fetching results in the loop, the cancellation of a running query is now
implemented for the postgres provider connection only because the GPKG
and GDAL both rely on GDALDatasetExecuteSQL which cannot be interrupted.
This PR also introduce a few optimizations in the PG DB-Manager
code that should probably fix also other "slowness" issues that
were reported after 3.x during PG query execution.
A small UX change in th SQL dialog makes it evident to the user that
a cancellation request has been sent to the backend: the button text
is changed to "Cancellation requested, please wait..." so that for
provider connections that are not able to interrupt the running query
and must wait for the fetching loop to exit from the exeuteSql call
the user knows that something is happening and that a cancellation
request has been successfully sent.
This is the implementation of the new DB connections API (grant proposal 2019).
Summary
The new API makes it available to QGIS core a new interface for provider connections and will allow to:
replace the provider specific QgsSettings management in QGIS4 (save/load connections from the settings) NOT IN SCOPE FOR NOW.
provide a unified API for common operations on DB connections:
executeSql and get the results
list tables names and properties and schemas
create a new vector table (no rasters for now)
create/rename/drop schemas and tables
vacuum
....
