Archiv der Kategorie: CBO

Issue with the Hint ENABLE_PARALLEL_DML

Performing an ETL with large data sets, it is often a good idea to run DML in parallel. But, in contrast to parallel query or DDL, parallel DML have to be explicitly enabled. You had to issue ALTER SESSION ENABLE PARALLEL DML in the past. Starting with 12c you can enable parallel DML specifically for each query using the hint ENABLE_PARALLEL_DML. For a few years now, I’ve been using the hint now and then and was quite happy. An observation I made a few days ago can lead to a rethinking. What I could observe is that for the SQL with embedded hint a new child cursor was created each time. Let’s test it! Weiterlesen

Online Statistics Gathering for ETL – Part 3

Preparing my session on Online Statistics Gathering for ETL for the DOAG conference, I noticed some points that I didn’t covered in the previous two blog posts. The first point is showing the problem that may arise if indexes are involved. The second one is about partition exchange load and it completes the topic of partitioned tables started in part 2. No blog posting on Oracle products is nowadays complete without mentioning the cloud. The third point is about Autonomous Data Warehouse Cloud Service and Online Statistics Gathering improvements. Weiterlesen

Subquery Unnesting – wenn es mal nicht funktioniert

Eigentlich leistet der CBO bei den „NOT IN“ Subqueries meistens einen guten Job und generiert effektive Ausführungspläne.
Es sei denn, irgendwas geht schief. Neulich war ich in einer Datenbank lange auf der Ursachenforschung und nur die CBO Trace-Dateien (Event 10053) haben mich letztendlich auf die richtige Spur gebracht. Aber eins nach dem anderen. Weiterlesen