ORA-1502 while skip_unusable_indexes=true

Question

OWB 9.2
We have one mapping that fills the target table in 2 stages:
in the first stage the table is filled with INSERT /*+ APPEND */ from several source tables;
the target table operator is then used as one of the source tables for the next INSERT /*+ APPEND */ into this target table itself.
OWB generates 2 set based INSERT statement for this mapping. The second INSERT (that also reads the target table) aborts with ORA-1502 (one of the indexes of the target table unusable). This is true, but in a premapping operator a ALTER SESSION SET SKIP_UNUSABLE_INDEXES=TRUE is issued.
Why does this error occur anyway? And why not in the first INSERT then?
Jaap.

Answer 1

This is an expected behaviour when you have a unique key. Updating a column with an unusable unique index could cause database integrity issues.
Take a look at Bug Nº 589199, Note:281329.1 and Note:111494.1 in Metalink.
I don't really think this is a Bug, it's database expected behaviour.
"A workaround is to use a hint in the SQL statement which causes the
optimizer not to select a plan which uses an unavailable index, or
to drop and recreate the index."
I believe that's what you should look for.
Regards,
Marcos