[postgis-users] Setting multiple columns in one UPDATE request
Stephen Woodbridge
stephenwoodbridge37 at gmail.com
Mon Sep 28 11:37:19 PDT 2020
Regina,
If I recall, a big part of the performance for large geometries is
untoasting them but there isn't much you can do about that. I'm not sure
if the untoasted geometries are cached or not, or if they get passed to
multiple functions in the same query or if they get untoasted for each
use. My memory is old on this, has it changed in the more recent releases?
-Steve W
On 9/28/2020 12:32 PM, Regina Obe wrote:
>
> 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
>
> Get BlueMail for Android <http://www.bluemail.me/r?b=16117>
>
> 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.
>
> Get BlueMail for Android
> <http://www.bluemail.me/r?b=16117>
>
> 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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