[QGIS-Developer] Data provider default values and default value clause

[Nyall Dawson]

On Fri, 24 Jan 2020 at 19:28, Alessandro Pasotti <apasotti at gmail.com> wrote:
>

> 1. From the docs it seems that the defaultValueClause() should ONLY return clauses (like sequences, functions, CURRENT_TIMESTAMP etc.) and should NOT return literal defaults.

Correct. However -- not all providers are completely well behaved in
this regard. Some of these are because of limitations on the provider
itself, e.g. there is no way to different literal defaults from
clause-type defaults on the associated backend. When we can't
differentiate the two, we err on the side of returning a
possible-literal value from defaultValueClause (and not the reverse
and accidentally return a non-literal value from defaultValue() ).

> 2. From the docs it seems that defaultValue() should return ONLY literal defaults and NOT functions, ::nextval and friends

Correct (and should ALWAYS be the case)

> 3. OGR provider does return the actual client-side calculated value when calling defaultValue() ONLY in case of literal defaults and CURRENT_TIMESTAMP, CURRENT_DATE and CURRENT_TIME, it returns the clause definition in all other cases, is this correct?

Yes - it's deliberate, even thought it slightly violates the above
answer to (2) in that for this provider we pre-calculate these
quasi-literal values on the qgis side.

> 4. postgres provider in case the property EvaluateDefaultValues is true does something more and send the clause to the server for evaluation, returning the calculated value, otherwise it returns the clause definition.

That's correct also, and by design. As per the warnings in the
defaultValue() docs, defaultValue calls are potentially dangerous to
make and should only ever be done when creating features (and in that
situation, only ever called once per field per feature). Otherwise
when EvaluateDefaultValues is true we run the risk of evaluating
sequences multiple times for a single feature, causing "gaps" in the
sequence.

> 5. What to return in case of no defaults? Depending on provider and field types, some implementations return a NULL (QVariant()), some others return a Python None.

Slight correction: QVariant() != Python NULL. Rather QVariant() ==
Python None, and QVariant( QVariant::Int ) == Python NULL. The c++ api
doesn't usually differentiate between the two and QVariant() is more
commonly used. (And I personally think in 4.0 we should completely
remove the PyQGIS NULL/QVariant( QVariant::Int ) distinctions -- they
add much complexity to code without compelling enough benefits).

Short answer: return QVariant() when no default literal exists.

> So, I'm confused about the expected behavior, if the documentation of defaultValue() and defaultValueClause() is correct then the provider implementations are (at least for some of them) wrong.

Hope that clarifies!

Nyall

>
> I'd like to hear other developer's opinion before proceeding with a fix for the a.m. providers.
>
> Thank you in advance!
>
> --
> Alessandro Pasotti
> w3:   www.itopen.it
> _______________________________________________
> QGIS-Developer mailing list
> QGIS-Developer at lists.osgeo.org
> List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer
> Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer