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:[email protected]]
*On Behalf Of *Marco Boeringa
*Sent:* Sunday, September 27, 2020 1:18 PM
*To:* [email protected]
*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
<[email protected] <mailto:[email protected]>> 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
<[email protected] <mailto:[email protected]>>
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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