[postgis-users] Configuration and performance of PostGIS
Shaozhong SHI
shishaozhong at gmail.com
Tue May 10 01:16:40 PDT 2022
Hi, IMRE,
How about add one column in your examples - a column called count? In
where clause do something like count < total number of rows of the table as
a method to stop the recursive query?
Regards,
David
On Tue, 10 May 2022 at 04:48, Imre Samu <pella.samu at gmail.com> wrote:
> > Alternatively, can we develop a test to tell whether Walk the Network
> produces a traversing line, a tree or a loop?
>
> PostgreSQL 14 has some enhancements:
> *- "The SQL-standard SEARCH and CYCLE options for common table
> expressions have been implemented."*
>
> see* "7.8.2.2. Cycle Detection"* in the doc ( lot of examples !!!! )
> https://www.postgresql.org/docs/14/queries-with.html#QUERIES-WITH-RECURSIVE
>
> and https://www.postgresql.org/docs/14/sql-select.html
>
> *"The optional CYCLE clause is used to detect cycles in recursive queries.
> The supplied column name list specifies the row key that is to be used for
> keeping track of visited rows. A column named cycle_mark_col_name will be
> added to the result column list of the WITH query. This column will be set
> to cycle_mark_value when a cycle has been detected, else to
> cycle_mark_default. Furthermore, processing of the recursive union will
> stop when a cycle has been detected. cycle_mark_value and
> cycle_mark_default must be constants and they must be coercible to a common
> data type, and the data type must have an inequality operator. (The SQL
> standard requires that they be Boolean constants or character strings, but
> PostgreSQL does not require that.) By default, TRUE and FALSE (of type
> boolean) are used. Furthermore, a column named cycle_path_col_name will be
> added to the result column list of the WITH query. This column is used
> internally for tracking visited rows. See Section 7.8.2.2 for examples."*
>
> and some examples
>
> https://www.depesz.com/2021/02/04/waiting-for-postgresql-14-search-and-cycle-clauses/
>
> regards,
> Imre
>
> Shaozhong SHI <shishaozhong at gmail.com> ezt írta (időpont: 2022. máj. 9.,
> H, 20:31):
>
>> Alternatively, can we develop a test to tell whether Walk the Network
>> produces a traversing line, a tree or a loop?
>>
>> Regards,
>>
>> David
>>
>> On Mon, 9 May 2022 at 18:42, <ruvenml at beamerbrooks.com> wrote:
>>
>>> The Walk the Network algorithm returns all points reachable from a
>>> particular starting point. The result is a tree. It only appears to be a
>>> single line because the network given as an example has been constructed
>>> without branches.
>>>
>>> Try adding:
>>> insert into network values ('linestring(3 4, 2 3)', 14);
>>>
>>> Note that what is returned is now a tree.
>>>
>>> Now, try adding:
>>> insert into network values ('linestring(0 0, 2 3)', 15);
>>>
>>> If you are patient enough, it will blow up with a memory allocation
>>> error because it creates a loop in the network.
>>>
>>> (Again, my appreciation to Paul Ramsey for constructing such a focused
>>> example.)
>>>
>>> Does a tree structure of paths starting at a designated node and ending
>>> at any node which has no outgoing edges satisfy your requirements or do you
>>> want the minimum cost/distance path? If so, you have lots of algorithms
>>> to choose from and watching some videos on graph theory might be time well
>>> spent.
>>>
>>> Ruven Brooks
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> On 5/9/2022 7:39 AM, Shaozhong SHI wrote:
>>>
>>> Hi, Imre,
>>>
>>> What happens if more than 1 result from the Walk the Network?
>>>
>>> Can recursive query return all possible results?
>>>
>>> How to handle such results?
>>>
>>> My guess that memory allocation error occurred because that more than 1
>>> result is found and the recursive query does not know what to do.
>>>
>>> What is your thought?
>>>
>>> Regards,
>>>
>>> David
>>>
>>> On Fri, 22 Apr 2022 at 22:14, Imre Samu <pella.samu at gmail.com> wrote:
>>>
>>>> > as St_intersects or recursive query used,
>>>>
>>>> The other alternative ( ~ less efficient ) is using a “noded”
>>>> network table ( "edge_table" )
>>>> in the recursive query. ( and don't forget to add indexes to the
>>>> "source" and "target" columns )
>>>>
>>>> WITH RECURSIVE walk_network(id, source, target, targetPoint) AS
>>>> (SELECT et.id,et.source,et.target,ST_EndPoint(the_geom) as
>>>> targetPoint
>>>> FROM edge_table et WHERE et.id = *12*
>>>> UNION ALL
>>>> SELECT e.id, e.source, e.target ,ST_EndPoint(the_geom) as
>>>> targetPoint
>>>> FROM edge_table e
>>>> , walk_network w
>>>> WHERE w.target = e.source
>>>> )
>>>> SELECT ST_AsText(ST_MakeLine(targetPoint))
>>>> FROM walk_network
>>>> ;
>>>> +---------------------------------+
>>>> | st_astext |
>>>> +---------------------------------+
>>>> | LINESTRING(4 2,3 2,2 1,1 1,0 0) |
>>>> +---------------------------------+
>>>> (1 row)
>>>>
>>>> regards,
>>>> Imre
>>>>
>>>>
>>>> Imre Samu <pella.samu at gmail.com> ezt írta (időpont: 2022. ápr. 22., P,
>>>> 16:39):
>>>>
>>>>> > With a large data set,
>>>>>
>>>>> :-)
>>>>> please give more detail:
>>>>> - How large?
>>>>> - and what is your real "business problem"? what type of network?
>>>>>
>>>>>
>>>>> > I tried to use this
>>>>> http://blog.cleverelephant.ca/2010/07/network-walking-in-postgis.html in
>>>>> the PostGIS.
>>>>>
>>>>> As I see this is a directed "network graph", and I will try using the
>>>>> pgRouting tool - for a large graph!
>>>>> *( "pgRouting extends the PostGIS/PostgreSQL geospatial database to
>>>>> provide geospatial routing and other network analysis functionality." )*
>>>>> The pgRouting project did not exist in 2010/07 when this blogpost
>>>>> was written!
>>>>>
>>>>> [image: image.png]
>>>>>
>>>>> so I have adapted the example network ( from the original blogpost )
>>>>> to pgRouting and this is my sample result
>>>>>
>>>>> ---------- ALL "downstream path" from "all deadends" sorted by
>>>>> descending cost ---------
>>>>>
>>>>> +------------+-----------+---------+-------------------------------------+--------------+
>>>>> | route_cost | start_vid | end_vid | the_geom_text
>>>>> | edge_ids |
>>>>>
>>>>> +------------+-----------+---------+-------------------------------------+--------------+
>>>>> | 6.24 | 3044 | 3000 | LINESTRING(4 4,3 4,2 3,1 2,1 1,0
>>>>> 0) | {13,9,6,3,1} |
>>>>> | 5.83 | 3043 | 3000 | *LINESTRING(4 3,4 2,3 2,2 1,1
>>>>> 1,0 0) | {12,8,5,2,1} |*
>>>>> | 4.83 | 3024 | 3000 | LINESTRING(2 4,2 3,1 2,1 1,0 0)
>>>>> | {10,6,3,1} |
>>>>> | 4.41 | 3014 | 3000 | LINESTRING(1 4,1 3,1 2,1 1,0 0)
>>>>> | {11,7,3,1} |
>>>>> | 3.41 | 3031 | 3000 | LINESTRING(3 1,2 1,1 1,0 0)
>>>>> | {4,2,1} |
>>>>>
>>>>> +------------+-----------+---------+-------------------------------------+--------------+
>>>>> and the second line is same as in the blogpost ( *"Downstream(12)" *example)
>>>>> ,
>>>>> just with an extra "deadends" points ; the edges :*
>>>>> {12,8,5,2,1} *
>>>>>
>>>>> start_vid : starting node/vertex id ( "deadends" in this example )
>>>>> end_vid : ending node/vertex id constant 3000 (0,0)
>>>>> node/vertex id = 3000 + X*10+Y coordinate // ( 2,1 ) --> 3021 ;
>>>>> (0,0) --> 3000
>>>>>
>>>>>
>>>>> > Whenever geospatial functions such as St_intersects or recursive
>>>>> query used,
>>>>>
>>>>> IMHO: A good scalable data model is extremely important.
>>>>> pgRouting has 2 important (separated) steps.
>>>>> - creating a routing topology - route optimized database ( with
>>>>> "start" - and "end" node/vertex )
>>>>> - fast routing/graph/"network-walking" functions - without the
>>>>> geometry ( using Boost Graph c++ library )
>>>>> ( in this example I have used
>>>>> https://docs.pgrouting.org/3.3/en/pgr_dijkstra.html )
>>>>>
>>>>>
>>>>> and this is my adapted "routing" topology edge table :
>>>>>
>>>>> DROP TABLE IF EXISTS edge_table CASCADE;
>>>>> CREATE TABLE edge_table (
>>>>> id bigint primary key,
>>>>> source bigint,
>>>>> target bigint,
>>>>> cost float,
>>>>> reverse_cost float,
>>>>> the_geom geometry
>>>>> );
>>>>> -- network example from
>>>>> --
>>>>> http://blog.cleverelephant.ca/2010/07/network-walking-in-postgis.html
>>>>> INSERT INTO edge_table VALUES( 1, 3011, 3000, 1, -1, 'LINESTRING(1 1,
>>>>> 0 0)');
>>>>> INSERT INTO edge_table VALUES( 2, 3021, 3011, 1, -1, 'LINESTRING(2 1,
>>>>> 1 1)');
>>>>> INSERT INTO edge_table VALUES( 3, 3012, 3011, 1, -1, 'LINESTRING(1 2,
>>>>> 1 1)');
>>>>> INSERT INTO edge_table VALUES( 4, 3031, 3021, 1, -1, 'LINESTRING(3 1,
>>>>> 2 1)');
>>>>> INSERT INTO edge_table VALUES( 5, 3032, 3021, 1, -1, 'LINESTRING(3 2,
>>>>> 2 1)');
>>>>> INSERT INTO edge_table VALUES( 6, 3023, 3012, 1, -1, 'LINESTRING(2 3,
>>>>> 1 2)');
>>>>> INSERT INTO edge_table VALUES( 7, 3013, 3012, 1, -1, 'LINESTRING(1 3,
>>>>> 1 2)');
>>>>> INSERT INTO edge_table VALUES( 8, 3042, 3032, 1, -1, 'LINESTRING(4 2,
>>>>> 3 2)');
>>>>> INSERT INTO edge_table VALUES( 9, 3034, 3023, 1, -1, 'LINESTRING(3 4,
>>>>> 2 3)');
>>>>> INSERT INTO edge_table VALUES(10, 3024, 3023, 1, -1, 'LINESTRING(2 4,
>>>>> 2 3)');
>>>>> INSERT INTO edge_table VALUES(11, 3014, 3013, 1, -1, 'LINESTRING(1 4,
>>>>> 1 3)');
>>>>> INSERT INTO edge_table VALUES(12, 3043, 3042, 1, -1, 'LINESTRING(4 3,
>>>>> 4 2)');
>>>>> INSERT INTO edge_table VALUES(13, 3044, 3034, 1, -1, 'LINESTRING(4 4,
>>>>> 3 4)');
>>>>>
>>>>> full example code - with data&code:
>>>>> https://gist.github.com/ImreSamu/efda6093b67391a0edafff39d8056cb5
>>>>>
>>>>> if you are interested in more examples.. check the pgRouting tutorial
>>>>> for example: *"Pre-processing waterways data"*
>>>>>
>>>>> https://workshop.pgrouting.org/2.7/en/un_sdg/sdg11-cities.html#pre-processing-waterways-data
>>>>>
>>>>> regards,
>>>>> Imre
>>>>>
>>>>>
>>>>> Shaozhong SHI <shishaozhong at gmail.com> ezt írta (időpont: 2022. ápr.
>>>>> 22., P, 1:22):
>>>>>
>>>>>> Whenever geospatial functions such as St_intersects or recursive
>>>>>> query used, the PostGIS appears to spawn away to many child queries and
>>>>>> just obliterate the CPU. Nothing finishes.
>>>>>>
>>>>>> That forced me to try out to do the some tasks on the FME server.
>>>>>>
>>>>>> I tried to use this http://blog.cleverelephant.ca/2010/07/network
>>>>>> -walking-in-postgis.html in the PostGIS.
>>>>>>
>>>>>> I tried to linecombiner in FME. LineCombiner | FME (safe.com)
>>>>>> <https://www.safe.com/transformers/line-combiner/>.
>>>>>>
>>>>>> With a large data set, the running of processors were monitored. It
>>>>>> was estimated the PostGIS one would take 16 days to complete.
>>>>>>
>>>>>> But, it only took a few minute to do the same thing in FME.
>>>>>>
>>>>>> This suggests that something is not right with the PostGIS Server.
>>>>>>
>>>>>> Have anyone got experience with configuration and improving
>>>>>> perfomance of PostGIS Server?
>>>>>>
>>>>>> Regards,
>>>>>>
>>>>>> David
>>>>>> _______________________________________________
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