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 <ma...@boeringa.demon.nl
<mailto:ma...@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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