[Qgis-user] Adding feature to PostGIS layer that has joins is really slow

[chris hermansen]

Nuno and list

On Sat, May 2, 2020 at 11:58 AM Nuno Lopes <nlopes at gmail.com> wrote:

> Hello, I have already posted this on
> https://gis.stackexchange.com/questions/360171/adding-feature-to-postgis-layer-that-has-joins-is-really-slow
> , just posting here to see if someone else has ideas.
>
> I have a large project that I've been setting up to work with QGIS 3.12
> and I have now hit a problem with using "joins" to join several attribute
> tables with a geometric one.
>
> I have reduced the problem to a minimal setup that has only two layers
> with a one to one (or zero) relation:
>
>  1. `point_layer` - a layer with a geometry (points) that has 64k entries
>  2. `attribute_layer` - a layer with one text field and a foreign key to
> the `point_layer`
>
> On the `point_layer` I added a join to the `attribute_layer` so that we
> can edit the attributes directly in the feature form.
>
> After doing this whenever I try to create a point on the `point_layer`
> **it takes more than 1 min to create the feature**.
>
> By examining the PostgreSQL log I found out that QGIS is querying each
> feature on the `point_layer` for it's relation on the `attribute_layer`.
> These queries are made one by one, so even though I'm running the database
> on my own computer, the sheer number of queries leads to very poor
> performance.
>
> I've already tried tweaking the cache config on the joins, hiding the form
> on add feature and enabling the "automatically create transaction groups
> where possible" to no avail.
>
> I've also tested older QGIS versions 3.4 and 3.8 without success.
>
> Am I doing anything wrong? Is there any way to work around this?
>
> The schema used is:
>
>     CREATE TABLE point_layer (
>         ogc_fid integer PRIMARY KEY,
>         geom public.geometry(GeometryZ,3763),
>         height double precision
>     );
>
>     CREATE INDEX point_layer_geom_idx ON point_layer USING gist (geom);
>
>     CREATE TABLE attribute_layer (
>         id SERIAL PRIMARY KEY,
>         fid integer REFERENCES point_layer(ogc_fid),
>         some_value text
>     );
>
> The query that is made repeatedly is (the fid changes):
>
>     SELECT "id","some_value"::text,"fid"::text
>     FROM "public"."attribute_layer"
>     WHERE ("fid" = ?) LIMIT 1
>
> The explain analyze is (the table is empty):
>
>                                                       QUERY PLAN
>
>
> ---------------------------------------------------------------------------------------------------------------
>      Limit  (cost=0.00..4.17 rows=1 width=68) (actual time=0.014..0.014
> rows=0 loops=1)
>        ->  Seq Scan on test_relation  (cost=0.00..25.03 rows=6 width=68)
> (actual time=0.011..0.011 rows=0 loops=1)
>              Filter: (fid = 29853)
>      Planning Time: 0.506 ms
>      Execution Time: 0.086 ms
>
>
>
> Why not just put your attributes on the same table as the spatial one?

If you don't like that idea you could consider putting an index on the fid
on the attribute_layer.  But it's odd to me to see the select on the
attribute_layer table given that your fid in your attribute_layer is a
foreign key referencing the ogc_fid field in point_layer.


-- 
Chris Hermansen · clhermansen "at" gmail "dot" com

C'est ma façon de parler.
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