[postgis-users] Setting multiple columns in one UPDATE request

Mon Sep 28 09:32:18 PDT 2020

The thing about CTEs is that in PostgreSQL 12+ they don’t necessarily materialize.  So the behavior should be the same CTE / Subquery (unless you throw in an OFFSET 0 which will absolutely force a materialization).  You can do that in subquery as well.

AS mentioned in my other note – ST_Area, ST_Perimeter are very low cost functions so I would expect no materialization for PG 12+ regardless if you use CTE or subquery.

From: postgis-users [mailto:postgis-users-bounces at lists.osgeo.org] On Behalf Of Marco Boeringa
Sent: Monday, September 28, 2020 3:37 AM
To: postgis-users at lists.osgeo.org
Subject: Re: [postgis-users] Setting multiple columns in one UPDATE request

Regina,

Thanks for your suggestion.

How is this performance wise? Is not using a CTE as in your suggestion, supposedly faster than with using a CTE, or is this just a syntax thing and performance is expected to be equal?

It would still be nice though, if PostgreSQL somehow handled this automatically, and one could use the most basic form yet be sure it was optimized. It also reads more easily to just see:

UPDATE <MY_TABLE> SET area = ST_Area(<GEOMETRY_COLUMN>), area_perimeter = ST_Area(<GEOMETRY_COLUMN>) / ST_Perimeter(<GEOMETRY_COLUMN>)

in your code, instead of more elaborate construct involving a join.

Marco

Op 28-9-2020 om 03:26 schreef Regina Obe:

I prefer doing it in the FROM and not bothering using a CTE.

So something like

UPDATE <MY_TABLE> SET area = f.area, area_perimeter = f.area/f.perimeter
FROM (SELECT id, ST_Area(<GEOMETRY_COLUMN>) AS area, ST_Perimeter(<GEOMETRY COLUMN>) AS perimeter
        FROM <MY TABLE> ) AS f
WHERE f.id = <MY TABLE>.id;

Note the FROM does not need to be the same as your table, you just need to have a common join.  That makes it particularly handy for updating with aggregate values 

From: postgis-users [mailto:postgis-users-bounces at lists.osgeo.org] On Behalf Of Marco Boeringa
Sent: Sunday, September 27, 2020 1:18 PM
To: postgis-users at lists.osgeo.org <mailto:postgis-users at lists.osgeo.org> 
Subject: Re: [postgis-users] Setting multiple columns in one UPDATE request

Thanks for pointing that out Alexander. I was just about to start a test after some code modifications, but now realized based on the example you pointed out that I was missing the "FROM cte" clause in my SQL statement. It feels a bit unnatural to have to specify that one, as you already define the cte name after the WITH keyword. But this example sorted it out.

Curious to see how it runs and if it leads to a measurable performance enhancement.

Marco

Op 27-9-2020 om 18:30 schreef Alexander Gataric:

You have the correct link. Here's an example.

https://stackoverflow.com/questions/36908495/update-with-result-from-cte-postgresql

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On Sep 27, 2020, at 7:47 AM, Marco Boeringa <marco at boeringa.demon.nl <mailto:marco at boeringa.demon.nl> > wrote: 

Hi Alexander, 

I guess I could use the optional WITH clause that is part of UPDATE in PostgreSQL?:

https://www.postgresql.org/docs/12/sql-update.html

It would have been nice to see an example of such usage in the PostgreSQL help, but I'll figure it out.

Marco

Op 27-9-2020 om 13:59 schreef Alexander Gataric:

Rewrite to have a CTE with the area calculations and join to the table.

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On Sep 27, 2020, at 2:29 AM, Marco Boeringa <marco at boeringa.demon.nl <mailto:marco at boeringa.demon.nl> > wrote: 

Hi all,

This may be an irrelevant basic question, but I just cannot find a clear 
answer to this, there is no documentation in the PostgreSQL docs for 
UPDATE about this:

In case I set multiple columns using some PostGIS function that clearly 
has a (considerable) cost associated with it, and the value of the first 
column being set is also needed to set the second column, does 
PostgreSQL automatically optimize this and re-use the value already 
calculated for column 1 to set column 2's value, or is each column's SET 
statement treated as independent entity?

E.g. let's say I want to calculate both area, and the division of area 
and perimeter, like:

UPDATE <MY_TABLE> SET area = ST_Area(<GEOMETRY_COLUMN>), area_perimeter 
= ST_Area(<GEOMETRY_COLUMN>) / ST_Perimeter(<GEOMETRY_COLUMN>)

Will PostgreSQL only process 'ST_Area(<GEOMETRY_COLUMN>)' once in this 
case, thus saving CPU load, or do I need to rewrite the SQL statement 
somehow to achieve that?

Marco

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