On 05/28/2014 10:30 AM, Hugo Mercier wrote:
Le 28/05/2014 08:35, Matthias Kuhn a écrit :
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.
Yes exactly. We have more or less the WHERE clause with expressions.
Aggregates, joins and subqueries could be built using something else
(relations?)
Well relations would need to be integrated into some form of syntax
first. And IMO this should be integrated nicely into the QgsExpression
system to avoid having yet another candidate for this thread. The output
of a relational query should be accepted as a node in a QgsExpression,
and it should be able to insert a QgsExpression as part of this a
relational query. So I think the best way is to extend expressions to
support iterators (and maybe single features?) as return values. But I
know that there are other opinions out there ;)
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.
AFAIK, you do not have direct access to the query sent to a virtual
table. But the SQLite engine will ask your virtual table driver what is
the best way to resolve a constraint on columns.
For instance, if the original query has a WHERE 'a = 2', then the driver
will be asked if it knows how to quickly resolve this, using indexes
(have a look at xBestIndex at http://www.sqlite.org/vtab.html)
So I guess, you could use the remote database indexes.
But, yes that would still be not very efficient if you want to query two
tables of the same postgis database.
In my mind, SQLite virtual tables are interesting for offering a more or
less relational view on data sources that are not originally designed
for that. But it would be suboptimal for already powerful databases.
We then have two kinds of "database backends" : real databases (postgis
and ... what else ?), and pseudo-databases through SQLite virtual tables.
And we would have a conversion from QgsExpression + relations to
different dialects of SQL (SQLite / PostgreSQL / ...) in each provider ?
Does it make sense for you ?
It does make sense.
Real Databases: PostgreSQL, Oracle, MSSQL, (sqlite, native SQL through OGR)
What would you like to use virtual tables drivers for?
The idea of having it available as another provider for layer
definitions sounds good to me anyway.
But should it also be possible to use sqlite syntax wherever we have
expression syntax integrated now?
Then the UI would be "Field / Expression / SQLite"?
In terms of control over what gets executed where and to translate it to
native SQL wherever possible.
In terms of quickly available functionality, sqlite virtual tables are
most likely an easy shot. Is there a possibility to overcome the problem
of "control"? Would be nice if yes, but I don't know the people behind
sqlite.
Is it worth adding two different syntaxes that may and will confuse
users for their advantages? In terms of usability I doubt that it is a
good idea (Apple would immediately fire any CTO who proposes having this
redundancy, but then, they have tons more development power than we have
:-) )
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)
Yes in my opinion, we should try to have the same code for what we call
"joins" and more general relations. And on the GUI part, some
redundancies between these two concepts may have to be reduced as well.
Sure thing. My idea is to allow some more control in the relations
window. For this purpose, I think a checkbox to "hard join" some fields
(doesn't need to be all, does it?) should do the trick I think.
Then we can also replace the fields properties "Join" dialog with a
"Relation" dialog that gives some control over relations that include
the current layer, so a user doesn't always have to resort to the
project properties.
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