Rewrite to have a CTE with the area calculations and join to the table. Get BlueMail for Android
On Sep 27, 2020, 2:29 AM, at 2:29 AM, Marco Boeringa <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 > >_______________________________________________ >postgis-users mailing list >postgis-users@lists.osgeo.org >https://lists.osgeo.org/mailman/listinfo/postgis-users
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