Question

I know I’m missing something obvious here. This trigger is updating all rows in the table (killing performance) when all I want it to do is perform the update on the new inserted row.

CREATE TRIGGER [dbo].[update_location_topo_name]
--fires at each row insert, queries topo map layer (must be present!) and inserts name of topo into new location record
on [dbo].[TBL_LOCATIONS] 
after insert
AS 
BEGIN 
update TBL_LOCATIONS
set TOPO_NAME = dbo.QD24K_GRSM.NAME
FROM dbo.tbl_locations
inner join dbo.QD24K_GRSM 
on TBL_LOCATIONS.Location_ID = TBL_LOCATIONS.Location_ID
WHERE  (QD24K_GRSM.Shape.STContains(TBL_LOCATIONS.SHAPE) = 1)  
END

Answer 1

You need to reference the INSERTED pseudo table to get only the rows that were inserted.

Additionally your join condition of TBL_LOCATIONS.Location_ID = TBL_LOCATIONS.Location_ID makes no sense at all.

Probably better to do this as an INSTEAD OF trigger to modify the rows before insert rather than after they are inserted.

CREATE TRIGGER [dbo].[update_location_topo_name] 
ON [dbo].[TBL_LOCATIONS] 
INSTEAD OF INSERT 
AS 
  BEGIN 
      INSERT INTO TBL_LOCATIONS 
                  (foo, 
                   bar, 
                   TOPO_NAME) 
      SELECT foo, 
             bar, 
             dbo.QD24K_GRSM.NAME 
      FROM   INSERTED I 
             LEFT JOIN dbo.QD24K_GRSM 
               ON QD24K_GRSM.Shape.STContains(I.SHAPE) = 1/* Will insert additional 
                                                    rows if more than one match*/
  END