Question

I have the following UPDATE statement:

update mytable
   set a = first_part(genid()),
       b = second_part(genid()),
       c = third_path(genid())
 where package_id = 10;

In this example the function genid() is called three times for each row, which is wrong – I want it to be called only once for each row of mytable.

I’m using PostgreSQL 8.4 database. How to write the correct update?

I’ve tried something like this:

update mytable
   set a = first_part(g),
       b = second_part(g),
       c = third_path(g)
 where package_id = 10
  from genid() as g;

But it didn’t work, because genid() has been called only once for the whole update statement.

Answer 1

Have you tried Postgres’ non-standard UPDATE .. FROM clause? I imagine, this would work

update mytable
   set a = first_part(gen.id),
       b = second_part(gen.id),
       c = third_path(gen.id)
  from (
          select genid() as genid, id
          from mytable 
          where package_id = 10
       ) gen
 where mytable.id = gen.id;
 --and package_id = 10 -- This predicate is no longer necessary as the subquery
                       -- already filters on package_id, as Erwin mentioned

Note that I’m forcing genid() to be called exactly once per record in mytable within the subselect. Then I’m self-joining mytable and gen using a hypothetical id column.
See the documentation here:

http://www.postgresql.org/docs/current/interactive/sql-update.html

This seems to have been introduced with Postgres 9.0 only, though. If that seems too complicated (i.e. not very readable), you can still resort to pgplsql as user Florin suggested here.