UPDATE query suggestions

Question

We have a need for an update query, but we're starting with a SELECT statment to make sure that the update will only change the required number of records. We started with:
select t1.act_num, t1.text || ' ' || (select text
                                        from BAC t2
                                       where comtype = 'Request'
                                         and type = 'A'
                                         and t1.act_num = t2.act_num) as appended_text
from BAC t1
where comtype = 'Result'
  and type = 'A'This query takes quite awhile to run, bringing back about 500 records every 30 seconds. The query plan isn't that complex. Full scan on "t1", which makes sense because 1/3 of the type 'A' records have a comtype of 'Result'. Then, there's an index-based retrieval for the "t2" records.
I can write the query another way, which runs in under 20 seconds, but it's format won't fit into an update query:
select t1.act_num, t1.text || ' ' || t2.text as appended_text
from BAC t1, BAC t2
where t1.act_num = t2.act_num
  and t1.comtype = 'Result'
  and t1.type = 'A'
  and t2.comtype = 'Request'
  and t2.type = 'A'I tried already and got a "No key-preserved table" error:
update (select t1.text as t1_text, t2.text as t2_text
          from BAC t1, BAC t2
         where t1.act_num = t2.act_num
           and t1.comtype = 'Result'
           and t1.type = 'A'
           and t2.comtype = 'Request'
           and t2.type = 'A')
   set t1_text = t1_text ||' '||t2_text;So does this mean it's just an update that is destined to be slow? The reason why I'm a bit confused as to the lethargy of the SELECT is that there are only about 350,000 records in this table, about 108,000 of which are actually being updated. It's just not that much data.
Thanks,
--=Chuck

Answer 1

Here's a new one for me ...
That act_num column was the 5th column in a 6-column primary key index. While the logic would have correctly updated the records like we wanted, Oracle would rather have more information. The following ran in a very acceptable time:
select t1.act_num, t1.text || ' ' || (select text
                                        from BAC t2
                                       where comtype = 'Request'
                                         and type = 'A'
                                         and t1.act_num = t2.act_num
                                         and t1.pk1 = t2.pk1
                                         and t1.pk2 = t2.pk2
                                         and t1.pk3 = t2.pk3
                                         and t1.pk4 = t2.pk4
                                      ) as appended_text
from BAC t1
where comtype = 'Result'
  and type = 'A'This is in Oracle 11gR1.
--=Chuck