<div dir="ltr"><div dir="ltr"><div dir="ltr">,Greetings,<div>I'm going over iterators for the prototype and it would appear that the OGR iterator uses by default the filtered fids (<a href="https://github.com/qgis/QGIS/blob/master/src/providers/ogr/qgsogrfeatureiterator.cpp#L48">https://github.com/qgis/QGIS/blob/master/src/providers/ogr/qgsogrfeatureiterator.cpp#L48</a>) </div><div><br></div><div>Though I'm not too sure about how this interacts with expression, maybe this is done in the core of the iterator, but nonetheless part of the work seem to already be done to some extent.</div><div><br clear="all"><div><div dir="ltr" class="gmail_signature"><div dir="ltr">Alexis Roy-Lizotte</div></div></div><br></div></div></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">Le mar. 30 avr. 2019 à 08:28, Matthias Kuhn <<a href="mailto:matthias@opengis.ch">matthias@opengis.ch</a>> a écrit :<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF">
<div class="gmail-m_426122242417419555moz-cite-prefix">Sorry, I lost you here.</div>
<div class="gmail-m_426122242417419555moz-cite-prefix"><br>
</div>
<div class="gmail-m_426122242417419555moz-cite-prefix">I think for both proposals three
QgsFeatureRequests will need to be created, requested and
iterators generated.</div>
<div class="gmail-m_426122242417419555moz-cite-prefix"><br>
</div>
<div class="gmail-m_426122242417419555moz-cite-prefix">Can you paste a minimal code sample
here that demonstrates the use of your proposed API?</div>
<div class="gmail-m_426122242417419555moz-cite-prefix"><br>
</div>
<div class="gmail-m_426122242417419555moz-cite-prefix">Thanks Matthias<br>
</div>
<div class="gmail-m_426122242417419555moz-cite-prefix"><br>
</div>
<div class="gmail-m_426122242417419555moz-cite-prefix">On 4/30/19 1:00 PM, Alexis R.L. wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">Hi,
<div><br>
</div>
<div>Well I could rework iterators to use the provided Fids as
the base list to iterate over, this would mean that instead of
checking every feature for each evaluation ( from what I'm
seeing each evaluation goes through every feature and check
for either the id number or if the expression is true.)</div>
</div>
</blockquote>
<blockquote type="cite">
<div dir="ltr">
<div>If we are doing 3 evaluation on 3 distinct symbols, this
mean that every feature will be iterated over 3 times. Because
each features are checked one by one for each of the 3
expression to evaluate.</div>
<div><br>
</div>
<div>By using Fids as the base list and incrementing the index
of it, each feature should be evaluated only once and no Fids
check would be necessary. (Assuming a simple legend with a
depth of one.)</div>
<div><br>
</div>
<div>This would drasticly speed up things when many loops are
required on a single feature. </div>
<div><br clear="all">
<div>
<div dir="ltr" class="gmail-m_426122242417419555gmail_signature">
<div dir="ltr">Alexis Roy-Lizotte</div>
</div>
</div>
<br>
</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">Le mar. 30 avr. 2019 à 06:30,
Matthias Kuhn <<a href="mailto:matthias@opengis.ch" target="_blank">matthias@opengis.ch</a>> a écrit :<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF">
<p>Hi,</p>
<p>An expression `attribute = 'major_road' AND $id IN
(1,2,3)` should have the same result on the number of
features in the iterator. Or I am missing something.<br>
</p>
<p>Matthias<br>
</p>
<div class="gmail-m_426122242417419555gmail-m_5060535740928964452moz-cite-prefix">On
4/30/19 12:13 PM, Alexis R.L. wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">Greetings Mr.Kuhn,
<div><br>
</div>
<div>Well in my case the fact that each symbol node is
evaluated add some lenght to the computation, if I can
make something that would only iterate over a given
list of Fids, (and at the same time prevent having to
iterate over each feature to check if that feature is
within the given list of feature) we would move from
something that takes x by n loops to something that
would only take around x loop ( where x is the number
of features and n is the number of symbol).</div>
<div><br>
</div>
<div>For other use case the gain might be minimal. Other
similar behaviour could be used with selected features
(though I'm not sure if this is a perfect match for
this also.) </div>
<div><br>
</div>
<div>This gain in efficiency is why I want to alter the
iterator itself rather than fiddling with expressions,
despite the fact that this is also a possibility but
far less optimal for the reason mentioned above.</div>
<div><br>
</div>
<div>I just want to know what would be the best way to
alter the iteration process without breaking and in a
way that could be leveraged.</div>
<div><br>
</div>
<div>Thanks.</div>
<div><br>
</div>
<div>
<div>
<div dir="ltr" class="gmail-m_426122242417419555gmail-m_5060535740928964452gmail_signature">
<div dir="ltr">Alexis Roy-Lizotte</div>
</div>
</div>
<br>
</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">Le mar. 30 avr. 2019
à 02:31, Matthias Kuhn <<a href="mailto:matthias@opengis.ch" target="_blank">matthias@opengis.ch</a>> a
écrit :<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF">
<p>Hi,</p>
<p>I like the idea of a nicer API to combine several
filter conditions. Currently the only possibility
to perform this operation is to write a filter
expression as a string or by combining expression
nodes.</p>
<p>I am not convinced however that this should be
done directly on QgsFilterRequest level. It feels
a bit like reimplementing the expression engine on
another level again.</p>
<p>Some questions that come to my mind:</p>
<p> * Are these filters always combined with AND or
also OR?</p>
<p> * Can these filters eventually be nested?<br>
</p>
<p> * How should we deal with compiling expressions,
should that be implemented again?</p>
<p>An alternative approach would be to work on a
nicer API for an expression builder to require 1)
less string manipulation when building an
expression and 2) have more introspection
capabilities when manipulating an expression.</p>
<p>Regards</p>
<p>Matthias<br>
</p>
<div class="gmail-m_426122242417419555gmail-m_5060535740928964452gmail-m_-619512251185950833moz-cite-prefix">On
4/29/19 9:02 PM, Alexis R.L. wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">Greetings Mr. Dawson,
<div><br>
</div>
<div>Thanks for the reply,</div>
<div><br>
</div>
<div>The tests that I did with the method in my
PR was provider agnostic but still entailed
looping over every feature to do the check.
Could you provide idea or methods that could
enable this behaviour in an API friendly way?</div>
<div><br>
</div>
<div>Should I make a new iterator entirely or
should I modify existing methods and add a
trigger like I tested in my PR?</div>
<div><br>
</div>
<div>Thanks!<br clear="all">
<div>
<div dir="ltr" class="gmail-m_426122242417419555gmail-m_5060535740928964452gmail-m_-619512251185950833gmail_signature">
<div dir="ltr">Alexis Roy-Lizotte</div>
</div>
</div>
<br>
</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">Le dim. 28
avr. 2019 à 19:46, Nyall Dawson <<a href="mailto:nyall.dawson@gmail.com" target="_blank">nyall.dawson@gmail.com</a>>
a écrit :<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">On Sun, 28
Apr 2019 at 07:18, Alexis R.L. <<a href="mailto:alroyliz0@gmail.com" target="_blank">alroyliz0@gmail.com</a>>
wrote:<br>
><br>
> Greetings Everyone,<br>
><br>
> While working on my PR (9648) I noticed
that expression filter and a list of feature
IDs are both considered the same thing cannot
be used as two filtering criteria.<br>
><br>
> I am aware that the intent was to only
have one element active to filter out features
other than the rectangle(? might be wrong on
that one).<br>
><br>
> What I am wondering if it would be a good
thing to have both co exist (mostly for my PR
as of now). A simply way to do so would be to
use the give feature ids as the list to
iterate over and check for the expression if
there is one.<br>
<br>
I also think this would be desirable. The
tricky part (well, one<br>
tricky part -- feature iterators are low level
stuff, lots of<br>
complexity there!) is avoiding any API
breakage while adding new<br>
methods to handle your use case.<br>
<br>
The other (SUPER) tricky bit is that any
change to feature requests<br>
usually requires accompanying changes to every
vector data provider in<br>
order to keep consistent behaviour across all
these providers. The<br>
same request should always give the same
result, regardless of the<br>
actual underlying data provider. The
consequence of this is that<br>
you'll need to be update the postgres, mssql,
oracle, OGR, memory,<br>
spatialite, wfs, arcgis feature server, db2,
and vector layer feature<br>
iterator code. You'll also need to add many
new unit tests to cover<br>
all the changes to the different providers.<br>
<br>
Definitely not a trivial change! Don't let
this put you off making it,<br>
but just be aware that this change would take
even a QGIS core<br>
developer a week or more of work to implement.<br>
<br>
Nyall<br>
<br>
<br>
<br>
><br>
> I assume that such a thing might be
implemented without touching the current
behaviour, otherwise one would have to remove
either the expression or the IDs in some case
when we want to override the current filtering
method.<br>
><br>
> Using multiple filtering method might be
better than only forcing one and would provide
more flexibility.<br>
><br>
> As I have a PR that is related to this (<a href="https://github.com/qgis/QGIS/pull/9816" rel="noreferrer" target="_blank">https://github.com/qgis/QGIS/pull/9816</a>
) I would like to address this point in it and
would like to have the discussion occur there
as to the best way to implement such a thing
without any breakage if people are not opposed
to having only one filtering method in
qgsfeaturerequest.<br>
><br>
> Thanks and have a nice day!<br>
><br>
> Alex<br>
>
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