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Bugs Related to SQL Plan Directives Pack and Unpack

SQL plan directives are a new concept introduced in version 12.1. Their purpose is to help the query optimizer cope with misestimates. To do so, they store in the data dictionary information about the predicates that cause misestimates. Simply put, the purpose of SQL plan directives is to instruct the database engine to either use dynamic sampling or automatically create extended statistics (specifically, column groups).

Since the database engine automatically maintains (e.g. creates and purges) SQL plan directives, in some situations it is necessary to copy the SQL plan directives created in one database to another one. This can be done with the help of the DBMS_SPD package.

Here are the key steps for such a copy:

Reconstructing oratab from the cluster registry

At the Accenture Enkitec Group we have a couple of Exadata racks for Proof of Concepts (PoC), Performance validation, research and experimenting. This means the databases on the racks appear and vanish more than (should be) on an average customer Exadata rack (to be honest most people use a fixed few existing databases rather than creating and removing a database for every test).

Nevertheless we gotten in a situation where the /etc/oratab file was not in sync with the databases registered in the cluster registry. This situation can happen for a number reasons. For example, if you clone a database (RMAN duplicate), you end up with a cloned database (I sincerely hope), but this database needs to be manually registered in the cluster registry. This is the same with creating a standby database (for which one of the most used methods is to use the clone procedure with a couple of changes).

The Broken Statistics: "parse count (total)" and "session cursor cache hits"

The values provided by the “parse count (total)” and “session cursor cache hits” statistics are subject to several bugs. And, what’s worse, for years Oracle didn’t care to fix it. This is my impression, at least.

Then, when few weeks ago I read in the Oracle Support note 13837105.8 (Bug 13837105 – statistics “parse count (total)” and “session cursor cache hits” miscounted) that the bug introduced in was fixed, I hoped that others bugs in this area were fixed as well.

Unfortunately, it’s not the case. What a disappointment!

Even though several posts were already wrote about this topic (e.g. here and here; check the comments as well…), I thought it could be useful to summarize what the current status is.

Named Notation and SEM_MATCH Table Function

This is a short post about a strange behavior I discovered today…

The fact is that it is not really possible to use the named notation with the SEM_MATCH table function. In fact, even though the parameter’s names can be specified, the order of the parameters overrides the specification done with the named notation!?!

Here is an example:

Gathering statistics in and encounters strange waits

It appears that, somewhere in the and patchsets, Oracle introduced some additional unwanted functionality to the “GATHER_*_STATS” procedures in the DBMS_STATS package.

I have been working on a customer’s database supporting a data-mart application where the data loading programs call DBMS_STATS.GATHER_TABLE_STATS as a concluding part of load processing, which means that the procedure gets called a *lot*.

We noticed that some calls to the same procedure were waiting excessively on the event “enq: TX – allocate ITL entry” and being blocked by sessions calling similar DBMS_STATS procedures, themselves in turn waiting excessively on the event “row cache lock” on data dictionary tables like SYS.CON$ (i.e. underlying the DBA_CONSTRAINTS view) and SYS.WRI$_OPTSTAT_SYNOPSIS_HEAD$.

How Many Children Can a Parent Cursor Have? 1,000,000?

The patch set includes a fix for bug# 10187168 which, in reality, is an enhancement request. Its purpose is to artificially limit the number of child cursors that a parent cursor can have. The concept is quite easy: when a parent cursor reaches _cursor_obsolete_threshold (default value is 100) child cursors the parent cursor is obsoleted and, as a result, a new one is created.

So, as of (or with some PSUs and bundle patches), the answer to the question is: 100.

Exchange Partition, Virtual Columns And Column Statistics

Here is an odd bug that can lead to some nasty side effects when using the EXCHANGE PARTITION technique. It is probably there for a very long time, simply because it depends on the usage of virtual columns, and the basic technique of virtual columns was introduced way back in the Oracle 8i times with the introduction of Function Based Indexes.

The problem isn't the exchange partition operation itself, but the accompanying swap of object statistics information, in particular the column statistics.

Look the following sequence of DDL and DML commands and pay then special attention to the output for the column statistics before and after the EXCHANGE PARTITION operation:

Event 10046 – Full List of Levels

Extended SQL trace (a.k.a. debugging event 10046 at a level higher than 1) is one of the key features provided by Oracle to troubleshoot applications using Oracle Database. For many years the available levels were always the same (4, 8 and 12). In fact, since I wrote my first paper about it in May 2000 and the release of 11g nothing changed.

With 11g, as I described in this post, new levels (16 and 32) were introduced.

Report Generators And Query Transformations

Usually the Cost-Based Optimizer arrives at a reasonable execution plan if it gets the estimates regarding cardinality and data scattering / clustering right (if you want to learn more about that why not watch my Webinar available at ""?).

Here is an example I've recently come across where this wasn't case - the optimizer obviously preferred plans with a significantly higher cost.

The setup to reproduce the issue is simple:

Obsolete cursors

In the previous post I wrote about strangely behaving V$SQL. For some reason there were duplicate rows leading to wrong results issue when running DBMS_XPLAN.DISPLAY_CURSOR for a particular child cursor. I tried to reproduce the issue using simple test case – and it was reproduced.

Here it is. I’m starting to execute a simple query against DUAL while constantly changing optimizer environment, forcing Oracle to build a new child cursor for each execution: