> -----Original Message----- > From: Sven Willenberger [mailto:sven@xxxxxxx] > Sent: Wednesday, February 01, 2006 2:13 PM > To: Justin Pasher > Cc: pgsql-general@xxxxxxxxxxxxxx > Subject: Re: [GENERAL] Best way to handle table trigger on update > > > On Tue, 2006-01-31 at 13:45 -0600, Justin Pasher wrote: > > Postgres 7.4.7 (I know, a little old, but we haven't had a chance to > > upgrade) > > > > I have a table that stores menu items for a side navigation > menu for a web > > site. Each menu item has a "position" column set that > determines where to > > put the menu item in the display. At any given time, the > menu items should > > not have any conflicting positions and should be > sequential. For example > > > > id | name | position > > -----+-------------------+---------- > > 1 | About Us | 1 > > 2 | History | 2 > > 3 | Support | 3 > > 4 | Job Opportunities | 4 > > 5 | Sitemap | 5 > > > > ... > > > > I have an UPDATE trigger defined on the table to handle keeping the > > positions correct. > > > > CREATE TRIGGER "update_menu_item" BEFORE UPDATE ON > "menu_items" FOR EACH ROW > > EXECUTE PROCEDURE update_menu_item(); > > > > When I update an existing row (say ID 3) with a new > position (let's say 1), > > the trigger will bump the menu items with a lower position up by one > > (position 2 becomes 3, 1 becomes 2) and everything is back > to normal. The > > catch is the trigger performs this position bumping by > making an update on > > the menu items table, thus firing the trigger again for > each updated row > > (and leading to chaos). Currently, the only workaround I > have found is to > > drop the trigger at the start of the stored procedure, make > the updates, > > then recreate the trigger. > > Rather than using a trigger why not create a function to do > the update? > The following will do the trick with the only modification needed to > your table is the addition of the boolean column "isupdate" > which should > default to false. The two arguments taken by the function are the > current position of the intended menu item and its new target > position: > > create or replace function update_menu_item(int,int) returns void as ' > update menu_items set isupdate = true where position = $1; > update menu_items set position = case when $1 > $2 THEN > position +1 when > $2 > $1 then position - 1 else position end > where position <= case when $1 > $2 then $1 else $2 end and > position >= > case when $1 > $2 then $2 else $1 end and isupdate = false; > update menu_items set position = $2 where position = $1 and isupdate; > update menu_items set isupdate = false where isupdate = true; > ' > LANGUAGE sql volatile; > > Then if you want to move Job Opportunities from position 4 to position > 2, just call the function: > select update_menu_item(4,2); > > HTH, > > Sven This would work, but my goal is to create something that is transparent to the user that is inserting the data (i.e. they perform a normal INSERT/UPDATE on the table and "It Just Works"). Thanks for the suggestion. Justin Pasher
