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Stats advisor

This is just a little shout-out about the Stats Advisor – if you decide to give it a go, what sort of things is it likely to tell you. The answer is in a dynamic performance view called v$stats_advisor_rules – which I’ve list below from an instance running

Lost time

Here’s a little puzzle that came up in the ODC database forum yesterday – I’ve got a query that has been captured by SQL Monitor, and it’s taking much longer to run than it should but the monitoring report isn’t telling me what I need to know about the time.

Here’s a little model to demonstrate the problem – I’m going to join a table to itself (the self join isn’t a necessary feature of the demonstration, I’ve just been a bit lazy in preparing data). Here’s a (competely truthful) description of the table:

Oracle vs. SQL Server Architecture

There are a lot of DBAs that are expected to manage both Oracle and MSSQL environments. This is only going to become more common as database platforms variations with the introduction of the cloud continue. A database is a database in our management’s world and we’re expected to understand it all.

Its not an easy topic, but I’m going to post on it, taking it step by step and hopefully the diagrams will help. Its also not an apple to apple comparison, so hopefully, but starting at the base and working my way into it with as similar as comparisons as I’m able to with features, it will make sense for those out there that need to understand it.

We have a number of customers that are migrating Oracle to Azure and many love Oracle and want to keep their Oracle database as is, just bringing their licenses over to the cloud. The importance of this is they may have Azure/SQL DBAs managing them, so I’m here to help.


A few months ago Franck Pachot wrote about a recursive SQL statement that kept appearing in the library cache. I discovered the note today because I had just found a client site where the following statement suddenly appeared near the top of the “SQL ordered by Executions” section of their AWR reports after they had upgraded to 18c.

select domain# from sys.im_domain$ where objn = :1 and col# = :2

I found Franck’s article by the simple expedient of typing the entire query into a Google search – his note was the first hit on the list, and he had a convenient example (based on the SCOTT schema) to demonstrate the effect, so I built the tables from the schema and ran a simple test with extended SQL tracing (event 10046) enabled.

SQL, PL/SQL and JavaScript running in the Database Server (Oracle MLE)

In a previous post I measured the CPU usage when running a database transaction in the same engine (SQL), or two engines in the same process (PL/SQL + SQL or JavaScript + SQL) or two processes (Javascript client + server SQL):

ODC Appreciation Day : Reduce CPU usage by running the business logic in the Oracle Database

For the JavaScript + SQL running in the same process, I used the Oracle Multi-Lingual Engine in beta 0.2.7 but there is now a new beta 0.3.0 and this post runs the same (or similar) with this.

I’ve installed this MLE in a previous post:

Oracle Multi-Lingual Engine

PI-Day example

Here is a very quick and easy test of the Oracle MLE, the engine that let you run JavaScript or Python stored procedures in the Oracle Database (currently in beta).

The MLE beta is provided as a docker image containing the Oracle Database 12.2 with the additional MLE libraries. I have created a VM on the Oracle Cloud in order to test it and show an end-to-end demo on Oracle Linux 7.

Get software

Here is where to download the database server with MLE beta:

Oracle Database MLE Download

and the SQLcl client

SQLcl Downloads

Oracle stored procedure compilation errors displayed for humans

Here is a script I use a lot especially when importing a schema with Data Pump and checking for invalid objects. I usually don’t care about compilation errors at compile time but just run UTL_RECOMP.RECOMP_PARALLEL at the end and check for errors on invalid objects. Here is an example.

I have imported a schema with Data pump and got some compilation errors:

I want to resolve them, or at least to understand them.

If I query DBA_ERRORS, I get the following:

This is a small example, but it can be huge. Not very helpful:

Hash Partitions

Here’s an important thought if you’ve got any large tables which are purely hash partitioned. As a general guideline you should not need partition level stats on those tables. The principle of hash partitioned tables is that the rows are distributed uniformly and randomly based on the hash key so, with the assumption that the number of different hash keys is “large” compared to the number of partitions, any one partition should look the same as any other partition.


I’ve made use of the function a few times in the past, for example in this posting on the dangers of using reverse key indexes, but every time I’ve mentioned it I’ve only been interested in the “leaf blocks per key” option. There are actually four different variations of the function, relevant to different types of index and controlled by setting a flag parameter to one of 4 different values.

The call to sys_op_lbid() take 3 parameters: index (or index [sub]partition object id, a flag vlaue, and a table “rowid”, where the flag value can be one of L, R, O, or G. The variations of the call are as follows:

Append hint

One of the questions that came up on the CBO Panel Session at the UKOUG Tech2018 conference was about the /*+ append */ hint – specifically how to make sure it was ignored when it came from a 3rd party tool that was used to load data into the database. The presence of the hint resulted in increasing amounts of space in the table being “lost” as older data was deleted by the application which then didn’t reuse the space the inserts always went above the table’s highwater mark; and it wasn’t possible to change the application code.

The first suggestion aired was to create an SQL Patch to associate the hint /*+ ignore_optim_embedded_hints */ with the SQL in the hope that this would make Oracle ignore the append hint. This won’t work, of course, because the append hint is not an optimizer hint, it’s a “behaviour” hint.