Why does DBMS_MVIEW.REFRESH have an implicit commit?

Question

I noticed recently that calling dbms_mview.refresh(...), which refreshes materialized views in Oracle, has an implicit commit.

Any ideas - other than "because it does" - why this action has an implicit commit?

Answer 1

According to Tom Kyte it is because a decision was made at design time to consider refreshing to be a DDL operation. Since all DDL operations implicitly commit, so does this one. Unfortunatly he doesn't answer the resulting question of why they choose to make it DDL.

Answer 2

Depending on your Oracle version and/or the parameters you supply dbms_mview.refresh may be doing a TRUNCATE followed by a direct load. TRUNCATE is a DDL command and as such issues an implicit commit. Direct load does not require a commit.

If you are using a more recent version of Oracle, I think 10.2+, you can set the atomic_refresh parameter to TRUE and it will refresh within a single transaction, using standard DELETE / INSERTs. This method could be quite a bit slower though.