Re: make view with union return one record

On 7/7/2010 8:27 AM, Merlin Moncure wrote:
> On Tue, Jul 6, 2010 at 3:43 PM, Andy Colson<andy@squeakycode.net>  wrote:
>> I have gis data in layers, and a pin might appear in either layer, or both
>> (parcelPoly, parcelPoint), or neither (and I dont care which I find)... so I
>> have this view:
>>
>>
>> create view howardia.getPoint(shapeid, x, y, acres, pin, extent) as
>>   SELECT gid,
>>         st_x(st_centroid(the_geom)) AS x,
>>         st_y(st_centroid(the_geom)) AS y,
>>         acreage,
>>         county_pin,
>>         st_box2d(st_expand(the_geom, 100))
>>   FROM howardia.parcelPoly
>>   UNION ALL
>>   SELECT gid,
>>         st_x(the_geom) AS x,
>>         st_y(the_geom) AS y,
>>         acreage,
>>         county_pin,
>>         st_box2d(st_expand(the_geom, 100))
>>   FROM howardia.parcelPoint;
>>
>> Which works fine for what I'm using.. it returns one or two records, and my
>> code just takes the first record and runs with it.
>>
>> but now... I'm adding something new, and having it return multiple records
>> per pin is causing problems.  I tried adding a limit inside the view but
>> then it never returns anything:
>
> ??? why not --  can you double check that?
>
> create view l as select * from (select 'a' union all select 'b') q limit 1;
>
> select * from l;
>   ?column?
> ----------
>   a
> (1 row)
>
> merlin

Humm.. yea, had to look at that a little closer... but no, it does not
work when you put a where clause on it:

gis=# create view l as select * from (select 'a' as cname union all
select 'b') q limit 1;
CREATE VIEW


-- This works as you noted:
gis=# select * from l ;
  cname
-------
  a
(1 row)


-- And as long as you only ever want the first row, it works too:
gis=# select * from l where cname = 'a';
  cname
-------
  a
(1 row)

-- But this is more akin to what I'm doing:
gis=# select * from l where cname = 'b';
  cname
-------
(0 rows)


In my example, if I put the limit on the view, then I can:

gis=# select x,y,pin from getpoint;
         x         |        y         |       pin
------------------+------------------+-----------------
  5185561.61663698 | 3906727.90184405 | 190161634020000
(1 row)

So the entire view is now just one record.

I'm guessing the limit and the where are being applied in the wrong
order.  (Correction, not the wrong order, rather not the order I want)

Hum... I wonder if I subselect the table enough times if I can swap the
where and the limit...  I'll have to play around with that.

-Andy