On 4/28/2011 2:34 PM, Dara Olson wrote:
Greetings. I am hoping that I have posted this on the most appropriate
list, please let me know if I should be posting to a different list.
In our Mapserver application, we join a lot of tables together -
generally one table with geometry to a "flat" table with tabular data.
My question is - is it more efficient/faster to create a view in
PostgreSQL doing the join and link to the view table from Mapserver or
to define the join (SQL statement) with Mapserver in the data definition
within the layer definition? Does it even matter because Mapserver sends
the request to PostgreSQL/GIS?
Also, we have joins that take a field within the flat table to determine
which table to join to get geometry. For example, if the flat table
column "join" value is equal to 1 then it joins to "table1" to get
geometry or if column "join" value is equal to 2 then it joins to
"table2" to get geometry. I read something that you were not supposed to
use WHERE clauses in the Mapserver data definition. Would this be better
to do as a view and link to the view table from the data definition?
Any advice would be greatly appreciated. Thanks in advance!
Dara
Dara,
I don't think it matter if you create views or not from a performance
point of view. View do allow you to hide much of the internals from the
mapfile but that might not matter.
The follow are some random examples of postgis DATA statements from some
of my mapfiles. If you want to do complex queries then you need to wrap
them into sub-query and that can have joins, where, union whatever you
need in it.
DATA "the_geom from (select gid, name, the_geom from lebanon_data.ai
union all select gid, name, the_geom from data.ai) as foo using unique
gid using srid=4326"
DATA "the_geom from (select gid, coalesce(key::text,'') as key,
feature, coalesce(st_type_abbr,' ') as st_type_abbr,
coalesce(civic_num,' ') as civic_num, coalesce(street_name,' ') as
street_name, the_geom from v_polygon) as foo using unique gid using
srid=2036"
DATA "the_geom from (select id, chk, the_geom from vertices_tmp where
chk=1 and the_geom && setsrid(!BOX!, 4326) ) as foo using SRID=4326
using unique id"
The !BOX! feature might be a custom patch I create or a standard
mapserver feature I can't remember off the top of my head and don't have
time at the moment to check into that, but what is does is substitute
the text to define a box representing the image extents so I can place
the in the subquery rather than the main query for performance reasons
as the vertices_tmp table is huge.
-Steve W
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