ST_Area and ST_Perimeter functions are relatively low cost, so that fact you discovered is not surprising.
I think if you were doing something like ST_Distance then the CTE or subquery would be more efficient than your direct solution. From: postgis-users [mailto:[email protected]] On Behalf Of Marco Boeringa Sent: Monday, September 28, 2020 10:15 AM To: [email protected] Subject: Re: [postgis-users] Setting multiple columns in one UPDATE request Hi Regina, I can now partially answer my question about performance myself: It turns out that for datasets having relatively small geometries (in terms of number of vertices, not area, e.g. a few dozen to a few hundred vertices maximum) there is actually *NO* benefit at all of rewriting the query either with a WITH (CTE) or FROM (Subquery). This may be different though for other datasets having much larger geometries, but needs further testing. In fact, processing is marginally slower, but only by 5-10% or so, compared to the original query. In my setup, I can also run the query both in a single thread, or using a custom Python multi-threaded implementation sending SQL statements in parallel to PostgreSQL. Since the test system has a very limited 4 core multi-threaded processor, the benefits of the multi-threading versus single threaded processing in this case are nil, obviously due to the overhead of the multi-threading. The multi-threaded application is as fast as the single threaded PostgreSQL worker, or even a bit slower, but puts a far higher load on the processor. Of course, with a more modern processor with high core count, this experience likely changes. There also appears to be virtually no difference between using a CTE or the subquery as you suggested: subquery is only very marginally faster than CTE. So for datasets having small geometries, just sticking to the original query like: UPDATE <MY_TABLE> SET area = ST_Area(<GEOMETRY_COLUMN>), area_perimeter = ST_Area(<GEOMETRY_COLUMN>) / ST_Perimeter(<GEOMETRY_COLUMN>) is fine for those datasets. I think this result is caused by the fact that the retrieving and storing overhead of the geometries (tables stored on SSD), is simply far bigger than the actual cost of calculating the area or perimeter for such datasets where the majority of geometries is of very limited size (e.g. OSM buildings, simple landuse polygons). Additionally, there may be an extra cost due to the needed join for the CTE and subquery statements. Lastly, the cost of running ST_Area and ST_Perimeter may just be to low as well. There may be other functions in PostGIS with a much higher computational cost that would show a benefit from rewriting the query. I will attempt to run a second benchmark using a dataset with much larger geometries though (some with well over > 10k vertices), to see if that gives the same result, and report back. There may be a difference, but we will see... Marco *** Dataset with small geometries (most < 200 vertices) ********* - Single-threaded using ORIGINAL QUERY: 8m45s - Single-threaded using SUBQUERY (FROM): 8m52s - Single-threaded using CTE (WITH): 9m13s - Multi-threaded using ORIGINAL QUERY: 9m27s - Multi-threaded using SUBQUERY (FROM): 9m44s - Multi-threaded using CTE (WITH): 9m50s ******************************************************* Op 28-9-2020 om 09:36 schreef Marco Boeringa: 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;
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