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ACS

You’ve probably heard about adaptive cursor sharing, and possibly you’ve wondered why you haven’t seen it happening very often on your production systems (assuming you’re running a modern version of Oracle). Here’s a little bug/fix that may explain the non-appearance.

MOS Doc ID 9532657.8 Adaptive cursor sharing ignores SELECTs which are not fully fetched.

This bug is confirmed in 11.2.0.1, and fixed in 11.2.0.3. The problem is that the ACS code doesn’t process the statistical information from the cursor unless the cursor reaches “end of fetch” – i.e. if you don’t select all the data in your query, Oracle doesn’t consider the statistics of that execution when deciding whether or not to re-optimise a statement.

First_rows hash

Just like my posting on an index hash, this posting is about a problem as well as being about a hash join. The article has its roots in a question posted on the OTN database forum, where a user has shown us the following execution plan:

Missing Filter

I see that Christian Antognini posted a note about an interesting little defect in Enterprise Manager a little while ago - it doesn’t always know how to interpret execution plans. The problem appears in Christians’ example when a filter subquery predicate is applied during an index range scan – it’s a topic I wrote about a few months ago with the title “filter bug” because the plan shows (or, rather, fails to show) a “missing” filter operation, which has been subsumed into the predicate section of the thing that would otherwise have been the first child of the filter operation – the rule of recursive descent through the plan breaks, and the ordering that OEM gives for the operations goes wrong.

Pitfalls of Using Parallel Execution with SQL Developer

[This post was originally published on 2012/02/29 and was hidden shortly thereafter. I’m un-hiding it as of 2012/05/30 with some minor edits.]

Many Oracle Database users like tools with GUI interfaces because they add features and functionality that are not easily available from the command line interfaces like SQL*Plus. One of the more popular tools from my experiences is Oracle SQL Developer in part because it’s a free tool from Oracle. Given SQL Developer’s current design (as of version 3.1.07.42), some issues frequently show up when using it with Oracle Databases with Parallel Execution. SQL Developer also contains a bug that exacerbates this issue as well.

Upgrades again

I’ve just spent a couple of days in Switzerland presenting seminar material to an IT company based in Delemont (about 50 minutes drive from Basle), and during the course I picked up a number of new stories about interesting things that have gone wrong at client sites. Here’s one that might be of particular interest to people thinking of upgrading from 10g to 11g – even if you don’t hit the bug described in the blog, the fact that the new feature has been implemented may leave you wondering where all your machine resources are going during the overnight run.

Subquery Factoring

Here’s an interesting little conundrum about subquery factoring that hasn’t changed in the recent (11.2.0.3) patch for subquery factoring. It came to me from Jared Still (a fellow member of Oak Table Network) shortly after I’d made some comments about the patch. It’s an example based on the scott/tiger schema – which I’ve extracted from the script $ORACLE_HOME/rdbms/admin/utlsampl.sql (though the relevant scripts may be demobld.sql or scott.sql, depending on version).

As usual I’ve used an 8KB block size, LMT with uniform 1MB extents, and no ASSM to hold the data. I won’t reproduce the code to generate the schema, just the versions of the query with, then without, subquery factoring:

Bug fixes

From MOS (Metalink) a search for “Patch Set – List of Bug Fixes by Problem” is a useful search, andother is “Availability and Known Issues”. Whenever you find some behaviour that looks like a bug, it’s worth checking the patch sets for the patches or release that are newer than the version that you’re running – you may find that your problem is a known bug with a patch that might be back-ported.

For ease of reference, here are some of the results I got (sorted in reverse order of version) from the searches; you will need a MOS account to follow the links:

Index size bug

Here’s a follow-up to a post I did some time ago about estimating the size of an index before you create it. The note describes dbms_stats.create_index_cost() procedure, and how it depends on the results of a call to explain plan. A recent question on the OTN database forum highlighted a bug in explain plan, however, which I can demonstrate very easily. I’ll start with a small amount of data to demonstrate the basic content that is used to calculate the index cost.

FBI Bug

Here’s a funny little optimizer bug – though one that seems to have been fixed by at least 10.2.0.3. It showed up earlier on today in a thread on the OTN database forum. We’ll start (in 9.2.0.8) with a little table and two indexes – one normal, the other descending.

Correlation oddity

This one’s so odd I nearly posted it as a “Quiz Night” – but decided that it would be friendlier simply to demonstrate it. Here’s a simple script to create a couple of identical tables. It’s using my standard environment but, apart from fiddling with optimizer settings, I doubt if there’s any reason why you need to worry too much about getting the environment exactly right.