Adding feature to PostGIS layer that has joins is really slow

Question

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?

UPDATE: As requested by vince here is the table schema:

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

Answer 1

This has now been solved in qgis 3.14 by closing this bug