[postgis-users] Configuration and performance of PostGIS

[Shaozhong SHI]

Any further information on source and target?

Can I understand that source means startpoint of a line, and target means
an endpoint of a line?

Regards,

David

On Sat, 23 Apr 2022 at 13:29, Shaozhong SHI <shishaozhong at gmail.com> wrote:

>
> 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
>>
>
> I will have a close look.
>
> With the use of  WHERE w.target = e.source, it probably avoided time
> consuming computation of ST_DWithin.
>
> Would WHERE w.target = e.source force the Postgres to use index-scan?
>
> If so, that might well be a more efficient way to do it.
>
> Regards,
>
> David
>
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