Re: Transaction and cascade problem

Glenn,

> I have postgres 7.1 set up with two tables (groups, users).  groups
> has 2 columns name, groupid where id is the primary key autoincrement
> and name is unique.  users has 3 columns name, userid, groupid.  I
> have a constraint on users which says that groupid must exists in the
> group table.  I also have a cascade delete on the groups table to
> delete any users that are in the group I am removing.  I regular (no
> transaction) mode everything works fine.  When I start a transaction
> and I add a group to the group table, then delete it before a commit
> or rollback I get the following error:
> 
> ERROR: triggered data change violation on relation "groups"

Yes.  For some technical reason ( I'm not clear on this ) you cannot
both add and delete the same row within a PostgreSQL transaction.  This
is a known issue in 7.1.x; I do not know if it is fixed in 7.2.  

In the couple of functions where I need to add rows, then drop them, I
have been taking the following steps:
1. Add new rows
2. When I'm done with the rows, flag them as invalid.
3. Put SQL in another, regularly-running (1/hour) function that deletes
all invalid-flagged rows.
Sort of a messy workaround, I know, but there it is.

-Josh


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