dbms_redefinition houskeeping


dbms_redefinition actually is a nifty, but powerful little toolkit that let’s you change table-definitions without actually locking the table in such a manner that it would prevent regular operations from being interrupted.

You can read loads about it in the Oracle documentation or in the wealthy library by Mr. Tim Hall.

housekeepingOne thing I noticed, and which I want to share here has lots to do with the house keeping that is automatically done by dbms_redefinition. Actually it talks about some of the bits it didn’t brush up after itself.

dbms_redefinition works using triggers and materialized views to help switch from your current active production table, via a so-called interim table, back to your shiny new, redefined production table. You can follow this beautifully by querying the dba_segments view along the way.
For this it obviously creates this materialized view and the other required components and it removes them after you finish your redefinition-trip. After all that is done, you can just remove your interim table and be done with it.

At least, that is what happened in most of the cases and is what you would expect!

Though, in some cases… it proved impossible to drop the interim table. To me this was somewhat scary… did the redefinition not finish, or did it not finish correctly?

What happened?

There was this table that I redefined. It had referential integrity constraints (aka. foreign key constraints) pointing towards it. Of course dbms_redefinition neatly created version of these to the interim table to be sure nothing went wrong.build-in-flight

When finishing redefinition (with dbms_redefinition.finish_redef_table) most of the interim bits and pieces are cleared away and you just have to drop your interim table manually (okay, we can discuss if this actually would / could / should be automated, but let’s leave that).

But… when you are then manually dropping this interim table (in a busy production system, I tend to want to be careful and just issue ‘drop table int_<tablename>‘. That does not work. dbms_redefinition “forgets” to remove these referential integrity constraints in the other tables (which are neatly names tmp$$_<constraintname>).
This than means either issue ‘drop table int_<tablename> cascade constraints‘, which is more then the basic ‘drop table‘ or find these constraints and remove them manually first:

select 'alter table '||owner||'.'||table_name||' drop constraint '||constraint_name||';'
from dba_constraints dc
where constraint_type='R'
and r_constraint_name in
(
select constraint_name
from all_constraints
where table_name = 'INT_<tablename>'
);
alter table <schema>.<foreign table> drop constraint TMP$$_<constraint name>;

I guess, personally, I would like dbms_redefinition to do this for me…

It’s smart enough! it created them!

Just a quick and additional note, setting ddl_lock_timeout to 30 or 60 for your session can actually help and prevent a lot of non-sense on a busy system.

Hope this helps someone sometime 😉


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