[pgrouting-users] What is the architecture of pgRouting and how can I make it work faster?

[Martin Lee]

On Wed, Aug 3, 2011 at 2:39 PM, Daniel Kastl <daniel at georepublic.de> wrote:

>
>
> On Wed, Aug 3, 2011 at 9:03 PM, Martin Lee <martinchlee at gmail.com> wrote:
>
>> [Small note: The author of this question is familiar with PostGIS, but is
>> not familiar with C language]
>>
>> I have a 'ways' table in PostGIS consisting of about 90,000 ways. When I
>> do a pgRouting shortest path query it takes about 2,5 seconds to get the
>> result.
>>
>
> This is really not many ways. It should be a lot faster.
>
>    - Did you check your tables have indices?
>
>
All the tables have indices. Are there any special requirements to
particular tables to have indices?


>    - Do you use bounding boxes not to load all the data (but in your case
>    it's such a small number of ways that even loading all shouldn't take long.)
>
>
>
>>
>> I was pretty unsatisfied with it (I would like it to be about 1 second for
>> my imagined user) so I tried to do a 'select * from ways' query in psql with
>> \timing enabled. It took 3 seconds to respond. When I did 'select name,
>> length, x1, y1, x2, y2 from ways' (that is all needed to build a graph in my
>> opinion) it took even more - 4 seconds to respond.
>>
>> Based on that, I assume that pgRouting's SQL queries are somehow
>> optimized.
>>
>> The questions:
>> 1. Is it possible to improve the speed of pgRouting queries?
>>
>
> The bottleneck is the number of ways you select from the table.
> You can improve your speed by selecting less road links.
>

I am sorry, but I don't really understand what 'road links' mean in
pgRouting parlance. There are vertices which are built from OSM nodes, there
are ways which are extracted from OSM multi-line ways. How road links are
represented and in which table?


> Anything else (like routing algorithms for example) might affect
> calculation time, but this is only very little compared to the data volume
> to load.
>
> One thing you could check is how your way ID's look like. Lower ID's are
> better, so you may want to renumber them if there are large ID's and large
> gaps between them. You can create a new id attribute for that.
>

Is the column name 'gid'? By the way, why is it called 'gid' and not 'id'?



>
> Daniel
>
>
>
>> 2. How is the actual shortest path calculation done? Is it anything like
>> the following:
>> - Load the whole ways/edges table into memory with one 'select * from
>> ways';
>> - Build a graph from that;
>> - Apply a C-based shortest path algorithm to the graph;
>> - Return a result into psql server?
>>
>>
>> Can any of these steps be improved anyhow?
>>
>> Thank you!
>>
>> _______________________________________________
>> Pgrouting-users mailing list
>> Pgrouting-users at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/pgrouting-users
>>
>>
>
>
> --
> Georepublic UG & Georepublic Japan
> eMail: daniel.kastl at georepublic.de
> Web: http://georepublic.de
>
> _______________________________________________
> Pgrouting-users mailing list
> Pgrouting-users at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/pgrouting-users
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.osgeo.org/pipermail/pgrouting-users/attachments/20110803/a05c6818/attachment.html