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How to use the expression based (edit: data defined) labeling with postgis?

I'm wondering, whats the best way to use expression based (edit: data defined) labels with postgis (I'm new to postgis). Just by adding new columns for x and y and so on, or by using a relational table with that columns? Or is there a other way?

EDIT: The image below shows the common way to use data defined labels for shape files: Add a column for x and y position for the label (and/or color, size, etc.) to the attribute table and set the correct field in the layer preferences. Now I would like to know if its possible to store the x and y (etc.) columns into a second relational table (with postgis). Because I would like to use the Data-Defined-Function without modifying the attribute table itself.

enter image description here

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  • This is a Carto CSS problem, and depends on the visualization tool, I believe. If you're using something like Tilemill to visualize and symbolize your data, Carto CSS can do this easily. Otherwise, I'm not sure what you mean by labeling PostGIS data...
    – Inactivated Account
    Commented Mar 7, 2014 at 16:06

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I'm presuming you have a PostGIS layer added in QGIS, and you've labeled the layer features using an expression. I'm going further and I'm using some color for the label and even a shadow:

enter image description here

Now, the question is how to relate all this with PostGIS. Well, you may save the style of the layer into the database:

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The style includes every custom setting you've defined for the label, together with the built expression.

In your PostGIS database a layer_styles will be automatically created, containing your layer default style:

enter image description here

This way, every time when you'll open the PostGIS layer, the label expression and its style will be automatically retrieved from the layer_styles table.

As you can see, there is no need to define additional columns or other relational table to achieve this.

EDIT1:

If you ever need to see the data inside the PostGIS layer style, just pay a visit to the PostGIS layer_styles, identify your layer saved style, and click the styleqml column. To properly read the entire xml style (almost 200 lines), you may select all the lines (Ctrl + A), copy it (Ctrl + C) and then paste it (Ctrl + V) into an empty text file.

enter image description here

As an alternative, you can perform the same thing from QGIS:

enter image description here

EDIT2:

If you are looking for a way to express the x and y coordinates for all the point features in a PostGIS table or a shapefile, then you may use do this:

enter image description here

And this is the result:

enter image description here

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  • I need the "data defined"-function for example for the x and y coordinates of my label (Layer Properties -> Placement -> Data Defined -> Coordinate...). Its not saved in the style file but thats what I wanted. My question was not clear enough.
    – MartinMap
    Commented Mar 8, 2014 at 10:05
  • Thanks. The x and y coordinate are saved in a manually created column and not in the stylefile. That means, I still have to create manually a column for x and y like I did when I was working with shapefiles, right? So there is no advantage in that point to use postgis instead of shapefiles.
    – MartinMap
    Commented Mar 9, 2014 at 11:26
  • I have to apologize! Thank you for your patience! I added a screenshot to my initial post and I hope that Im clear enough now.
    – MartinMap
    Commented Mar 10, 2014 at 19:25

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