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September 2016

Basicfile LOBs 6

One of the nice things about declaring your (basicfile) LOBs as “enable storage in row” is that the block addresses of the first 12 chunks will be listed in the row and won’t use the LOB index, so if your LOBs are larger than 3960 bytes but otherwise rather small the LOB index will hold only the timestamp entries for deleted LOBs. This makes it just a little easier to pick out the information you need when things behave strangely, so in this installment of my series I’m going to take about an example with with storage enabled in row.

Basicfile LOBS 5

At the end of the last installment we had seen a test case that caused Oracle to add a couple of redundant new extents to a LOB segment after one process deleted 3,000 LOBs and another four concurrent processes inserted 750 LOBs each a few minutes later (after the undo retention period had elapsed). To add confusion the LOBINDEX seemed to show that all the “reusable” chunks had been removed from the index which suggests that they should have been re-used. Our LOB segment started at 8,192 blocks, is currently at 8,576 blocks and is only using 8,000 of them.

How will things look if I now connect a new session (which might be associated with a different freepool), delete the oldest 3,000 LOBs, wait a little while, then get my original four sessions to do their concurrent inserts again ? And what will things look like after I’ve repeated this cycle several times ?

Space Usage

Here’s a simple script that I’ve used for many years to check space usage inside segments.  The comment about freelist groups may be out of date  – I’ve not had to worry about that for a very long time. There is a separate script for securefile lobs.

How much memory is truly used by my Oracle instance?

There are many posts about the amount of memory that is taken by the Oracle database executables and the database SGA and PGA. The reason for adding yet another one on this topic is a question I recently gotten, and the complexities which surrounds memory usage on modern systems. The intention for this blogpost is to show a tiny bit about page sharing of linux for private pages, then move on to shared pages, and discuss how page allocation looks like with Oracle ASMM (sga_target or manual memory).

The version of linux in this blogpost is Oracle Linux 7.2, using kernel: 4.1.12-37.6.3.el7uek.x86_64 (UEK4)
The version of the Oracle database software is 12.1.0.2.160719 (july 2016).

Installing Pibrella on Raspberry Pi 3 and GPIO Pins

With me traveling to Redwood City next week to visit HQ and then Oracle Open World the week after, I’m busy during many of my off hours preparing a new Raspberry Pi STEAM setup for a maker’s faire the beginning of October.

How to resolve media failures with the Recovery Advisor in #Oracle

Error messages are showing up because files have been damaged? Database Recovery Advisor to the rescue!

Partitioning an existing index

I had a question on AskTom recently, where due to concurrency issues, the poster of the question wanted to take their existing primary key index and hash partition it to reduce the “hotness” of the leading leaf block(s).  Their database was 11g, but I thought it would worth showing off some 12c features that would let you do this process with minimal disruption.

First we’ll create our table, with a standard primary key index


drop table t purge;

create table t as select * from dba_objects
where object_id is not null;

create unique index IX on T ( object_id );

alter table T add constraint T_PK primary key ( object_id );

Now the task is to partition the index. In 12c, you can have multiple indexes defined on the same column(s), as long as only one of those indexes is visible. So we’ll create our new partitioned index invisible in online mode.

Video Tutorial: XPLAN_ASH Active Session History - Part 10

The next part of the video tutorial explaining the XPLAN_ASH Active Session History functionality continuing the actual walk-through of the script output.

More parts to follow.

Oracle 12c: Indexing JSON in the Database Part III (Paperback Writer)

In Part I and Part II, we looked at how to index specific attributes within a JSON document store within an Oracle 12c database. But what if we’re not sure which specific attributes might benefit from an index or indeed, as JSON is by it’s nature a schema-less way to store data, what if we’re not entirely sure […]

MemSQL — my next adventure

I have been working with Oracle database performance for many years now, but for me it is time to make a change.  Today I start with an exciting new and nimble company MemSQL.

MemSQL offers a new approach to real time analytics with lock-free data structures, in-memory row store, and column store.   MemSQL can perform Real-Time analysis as well as pull data from traditional EDW or Hadoop.