Hi All,
As the responsible person for QGIS relations, I feel obliged to share my
thoughts in this discussion.
First of all a few notes about QGIS expressions and their current state:
QGIS expressions are a nice and handy functionality to quickly calculate
values based on a single feature in QGIS. This serves a lot of use-cases
quick and easy. However, in their current state, they don't allow to do
exactly what this thread originates from: aggregate and join data from
other layers or use subqueries.
I like the idea of using sqlites virtual tables to have immediate access
to a huge base of functionality offered by sqlite to do complex queries.
If we introduce this support, we have immediate support for a wide range
of database functionality with a few lines of code. The only thing I am
not sure (and I don't know the virtual tables implementation enough to
answer this question) is if it is able to delegate cross-table queries
to the original database. In short: can I do a request that requires
data from different tables of the same database and have it executed
directly inside the database? My suspicion is no, because the sqlite
virtual tables will be known to sqlite as QGIS tables and it will still
query the tables through the QGIS provider, therefore calculating e.g. a
max functionality by querying the QGIS provider for all features and
then calculating the "max" locally and not on the server side. This
would be a major performance impact for customers having a single
database that could do this calculation for us instead of doing this
ourselves.
Instead, if we have QGIS expressions (or a QGIS query implementation on
top of it) support for this, we are able to tell, if different tables
are from the same database and therefore if we are able to delegate the
whole join/aggregate/subquery whatever job to the database and let the
database do what it's good at.
Therefore my question to the folks who know the sqlite virtual table
code: is it possible to have sqlite virtual tables forward cross table
queries to the database itself? Or is it possible to get access to the
parsed query tree (or whatever the name of that may be) to determine
based on QGIS side based on the parsed query if we are able to optimize
by forwarding to the database.
Concerning joins, merging this code with the relations seems a viable
option for me in the long run. Currently, relations do no caching and
do not "hard join" the data on the other table (meaning, joined fields
are not available as fields) and maybe there are other things missing
(I remember a thread here on the ML about this before)
Kind regards,
Matthias
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