This exposes some basic temporal capabilities for vector layers:
- static time range for layer (to match raster layer possibilities), this
sets a single static time range which applies to the whole layer. ALL
features from the layer will be shown whenever the canvas time
overlaps the layer time range
- "Single field with datetime": Allows selection of a single date
or datetime field from the layer. Features will be shown whenever
this field value is within the canvas time range
- "Separate Fields for Start and End Date/Time": Allows selection
of start and end date/datetime fields from the layer. Features will
be shown whenever the time interval calculated from these fields
overlaps the canvas time range
Some known limitations/inefficiencies:
- currently only date/datetime fields can be used. This was done
to simplify the format handling and avoid the need to worry about
string fields with different datetime formats. In future we should
allow selection of string fields and allow users to enter a custom
datetime format string
- unlike the Time Manager plugin approach, the approach taken here
is to rely completely on QGIS expressions and feature requests to
do the filtering (Time Manager uses layer filter strings and attempts
to set a native SQL filter syntax so that filtering is done on the
backend). This is intentional, because it provides a unified filter
approach regardless of the provider used (i.e. we don't need to worry
about the different SQL syntaxes used natively by the different
providers). The beauty of feature request expression compilation
**should** mean that the QGIS expressions are magically turned into
native backend queries, BUUUUUUUUUUUT... because we lack QGIS expression
support for date time literals, we currently rely on the "to_datetime"
expression function and coerce everything through strings. None of
the expression compilers handle this function, so currently ALL
filtering is done on the QGIS side. We need to add functions for
optimised datetime literal creation, and then ensure that the different
compilers correctly map these literals across to the backend
filter syntax to allow all the filtering work to be done on the database
side...
So currently, performance is much worse with large layers compared
to Time Manager. But, the advantage is that we can use the native
temporal framework and have vector layers animated alongside mesh
and raster layers!
