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12c

Storing Date Values As Characters Part II (A Better Future)

In the previous post, I discussed how storing date values within a character data type is a really really bad idea and illustrated how the CBO can easily get its costings totally wrong as a result. A function-based date index helped the CBO get the correct costings and protect the integrity of the date data. During […]

Storing Date Values As Characters (What’s Really Happening)

For something that’s generally considered an extremely bad idea, I’ve lost count of the number of times I’ve come across applications that insist on storing date values as characters within the database. We’ve all seen them … I recently got called in to assist a customer who was having issues with a POC in relation […]

Virtual Partitions

Here’s a story of (my) failure prompted by a recent OTN posting.

The OP wants to use composite partitioning based on two different date columns – the table should be partitioned by range on the first date and subpartitioned by month on the second date. Here’s the (slightly modified) table creation script he supplied:

library cache lock on BUILD$ object

I was testing an application performance in 12c, and one job was constantly running slower than 11g. This post is to detail the steps. I hope the steps would be useful if you encounter similar issue.

Problem

In an one hour period, over 90% of the DB time spent on waiting for library cache lock waits. Upon investigation, one statement was suffering from excessive waits for ‘library cache lock’ event. We recreated the problem and investigated it further to understand the issue.

Following is the output of wait_details_rac.sql script (that I will upload here) and there are many PX query servers are waiting for ‘library cache lock’ wait event.

Cursor_Sharing problem

Here’s a possible bug (though maybe “not a bug”) that came up over the weekend on the OTN database forum. An application generating lots of “literal string” SQL was tested with cursor_sharing set to force. This successfully forced the use of bind variable substitution, but a particular type of simple insert statement started generating very large numbers of child cursors – introducing a lot of mutex waits and library cache contention. Here’s a (substituted) statement that was offered as an example of the problem:

DDL logging

I was presenting at the UKOUG event in Manchester on Thursday last week (21st April 2016), and one of the sessions I attended was Carl Dudley’s presentation of some New Features in 12c. The one that caught my eye in particular was “DDL Logging” because it’s a feature that has come up fairly frequently in the past on OTN and other Oracle forums.

So today I decided to write a brief note about DDL Logging – and did a quick search of my blog to see if I had mentioned it before: and I found this note that I wrote in January last year but never got around to publishing – DDL Logging is convenient, but doesn’t do the one thing that I really want it to do:

TABLE Operator with Locally Defined Types

postit-missed-itIn my OUG Ireland 2016 – Summary post I mentioned the Oren Nakdimon session called “Write Less (Code) with More (Oracle 12c New Features)”. One of the things he mentioned was the removal of restrictions associated with the use of the TABLE operator on local table types. If I had read about this or seen it before, it had certainly slipped my mind, so I made a note to write something about it and add a link to it from my PL/SQL new features article. So here it is.

Set Operations

A recent post on the OTN database forum highlights a couple of important points ideas for optimising SQL. There are: (a) is there a logically equivalent way of stating the SQL and (b) is there a different “natural language” way of posing the problem.

The posting starts with a query, part of an execution plan, and a request to “get rid of the tablescan”. I guessed originally that the query came from an 11g instance, and the OP gave us some code to create the tables and indexes, so I’ve modelled the tables to get the indicated plan (then filled in the original numbers). This is the query, and my cosmetically adjusted version of the plan output that the OP probably got:

Index Usage

There are some questions about Oracle that are like the mythical Hydra – you think you’ve killed it, but for every head you cut off another two grow. The claim that “the optimizer will switch between using an index and doing a tablescan when you access more than X% of the data” re-appeared on the OTN database forum a little while ago – it doesn’t really matter what the specific value of X was – and it’s a statement that needs to be refuted very firmly because it’s more likely to cause problems than it is to help anyone understand what’s going on.

OTNYathra2016

Over the last many years, some of you have invited me to attend conferences in India, and talk about Oracle RAC and performance. I have not had an opportunity to make it to conferences in India, until now:)

I am excited to announce that I will be participating in OTN sponsored Oracle ACE Director’s tour in India (April 23rd to May 2nd 2016), and presenting ( with deep dive demos ) about RAC, performance, and in-memory. This is a golden opportunity for you to learn some of the internal stuff that I talk about in my class too.

Refer http://otnyathra.com for further details.