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SLOB Can Now Be Downloaded From GitHub.

This is a quick blog entry to announce that the SLOB distribution will no longer be downloadable from Syncplicity. Based on user feedback I have switched to making the kit available on GitHub. I’ve updated the LICENSE.txt file to reflect this distribution locale as authorized and the latest SLOB version is 2.4.2.1.

Please visit kevinclosson.net/slob for more information.

A look into oracle redo, part 10: commit_wait and commit_logging

The redo series would not be complete without writing about changing the behaviour of commit. There are two ways to change commit behaviour:

1. Changing waiting for the logwriter to get notified that the generated redo is persisted. The default is ‘wait’. This can be set to ‘nowait’.
2. Changing the way the logwriter handles generated redo. The default is ‘immediate’. This can be set to ‘batch’.

There are actually three ways these changes can be made:
1. As argument of the commit statement: ‘commit’ can be written as ‘commit write wait immediate’ (statement level).
2. As a system level setting. By omitting an explicit commit mode when executing the commit command, the setting as set with the parameters commit_wait (default: wait) and commit_logging (default: immediate).
3. As a session level setting. By omitting an explicit commit mode, but by setting either commit_wait or commit_logging it overrides the settings at the system level.

Covering indexes in Oracle, and branch size

A covering index is an index that contains all the columns required by your query, so that you don’t have to do a TABLE ACCESS BY INDEX ROWID, which is the major cost of an index range scan. You don’t need any special feature to do that in Oracle. Just add the required columns at the end of the index. In the execution plan you will see the columns used as index keys for the range scan displayed in ‘access’ predicates, and the further filtering done on the remaining columns with ‘filter’ predicates. The ‘projection’ shows the columns that are returned in the rowset result.
However you may have seen that SQL Server has a special ‘INCLUDING’ keyword to separate those non-key columns added only for filtering or projection but not for access. What does it bring that Oracle doesn’t have?

exp catch

No-one should be using exp/imp to export and import data any more, they should be using the datapump equivalents expdp/impdp – but if you’re on an older (pre-12c) version of Oracle and still using exp/imp to do things like moving tables with their production statistics over to test systems then be careful that you don’t fall into an obsolescence trap when you finally upgrade to 12c (or Oracle 18).

exp/imp will mess up some of your histograms if you’re still using them to move tables/statistics in 12c.

A look into oracle redo, part 9a: commit – concurrency considerations

During the investigations of my previous blogpost about what happens during a commit and when the data becomes available, I used breaks in gdb (GNU debugger) at various places of the execution of an insert and a commit to see what is visible for other sessions during the various stages of execution of the commit.

However, I did find something else, which is very logical, but is easily overlooked: at certain moments access to the table is blocked/serialised in order to let a session make changes to blocks belonging to the table, or peripheral blocks like undo, for the sake of consistency. These are changes made at the physical layer of an Oracle segment, the logical model of Oracle says that writers don’t block readers.

SQL Monitor

I’ve mentioned the SQL Monitor report from time to time as a very useful way of reviewing execution plans – the feature is automatically enabled by parallel execution and by queries that are expected to take more than a few seconds to complete, and the inherent overheads of monitoring are less than the impact of enabling the rowsource execution statistics that allow you to use the ‘allstats’ format of dbms_xplan.display_cursor() to get detailed execution information for a query. The drawback to the SQL Monitor feature is that it doesn’t report predicate information.

Lady Coders Conference, Denver 2017

This weekend I’m not going to have to say, “No, this is not my husband’s code” for a change.

Whitepaper Announcement: Migrating Oracle Database Workloads to Oracle Linux on AWS

This is just a quick blog entry to share a good paper on migrating Oracle Database workloads to Amazon Web Services EC2 instances running Oracle Linux.

Please click the following link for a copy of the paper:  Click Here.

 

A look into oracle redo, part 9: commit

The previous blogpost talked about a simple insert, this blogpost investigates what happens when the DML is committed. Of course this is done with regular commit settings, which means means they are not touched, which means commit_logging is set to immediate and commit_wait is set to wait as far as I know. The documentation says there is no default value, and the settings are empty in all parameter views. In my humble opinion, if you must change the commit settings in order to make your application perform usable with the database, something is severely wrong somewhere.

This blogpost works best if you thoroughly gone through the previous post. I admit it’s a bit dry and theoretical, but you will appreciate the knowledge which you gained there, because it directly applies to a commit.

First let’s look at the flow of functions for the commit:

After IoT, IoP makes its way to the database

At each new Oracle version, I like to check what’s new, not only from the documentation, but also from exposed internals. I look (and sometimes diff) on catalog views definitions, undocumented parameters, and even the new C functions in the libraries. At last Oak Table World, I was intrigued by this V$SQLFN_METADATA view explained by Vit Spinka when digging into the internals of how execution plans are stored. This view has entries with all SQL functions, and a VERSION column going from ‘V6 Oracle’ to ‘V11R1 Oracle’. The lastest functions has an ‘INVALID’ entry and we also can see some functions with ‘SQL/DS’. Well, now that we have Oracle 18c on the Oracle Cloud, I came back to this view to see if anything is new, listing the highest FUNC_ID at the top and the first row attired my attention: