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I’ve just been prompted to complete and publish a draft I started a few years ago. It’s (ultimately) about a feature that appeared in 9i but doesn’t seem to show up very often at client sites or as a common solution to performance problems on the various Oracle forums – but maybe that’s not surprising given how slowly analytic functions have been taken up.

I want to work towards the feature by starting with a requirement, then examine several solutions. To supply a touch of realism I’ll create an orders table, which holds a customer id and an order date (including time), ,and then ask for a report of the most recent order for each customer. Here’s some starting data:

Nulls are not stored in indexes … most of the time

This question got posed on Twitter today

image

I answered briefly on Twitter, but I thought I should give it some “meat” with a blog post.

Firstly, lets explore the commonly understood mechanism where in a conventional index, nulls are not stored and hence you can have multiple index entries (so to speak) where the value is null.

Here is my simple table T with a standard (ascending) unique index, and we can see immediately that there is no limit to the number of null values, where by “null” I mean that all indexed columns are null, that you can have in the table.

Add ORDER BY to make ANY query faster

Yes it’s SCBT day here in Perth!

SCBT = Silly Click Bait Title Smile

This post is just a cautionary tale that it is easy to get caught up judging SQL performance solely on a few metrics rather than taking a more common sense approach of assessing performance based on the true requirements of the relevant component of the application.  I say “true requirements” because it may vary depending on what is important to the application for a particular component.

Quiz Night

Here’s a question prompted by a recent thread on the ODevCom database forum – how many rows will Oracle sort (assuming you have enough rows to start with in all_objects) for the final query, and how many sort operations will that take ?


drop table t1 purge;

create table t1 nologging as select * from all_objects where rownum < 50000;

select owner, count(distinct object_type), count(distinct object_name) from t1 group by owner;

Try to resist the temptation of doing a cut-n-paste and running the code until after you’ve thought about the answer.

Rowids

I have, in the past, used the dbms_rowid package to create rowids from block addresses (typically faking the first and last rowids that could appear in an extent); but I’ve just been sent a piece of information by Valentin Nikotin that’s going to make me go back and check whether what I’ve done with the package will always give me the correct results. Here’s a little demonstration code that highlights the issue:

Distributed Sets

In an earlier post I’ve described how a distributed query can operate at a remote site if it’s a simple select but has to operate at the local site if it’s a CTAS (create as select) or insert as select. There’s (at least) one special case where this turns out to be untrue … provided you write the query in the correct fashion. I discovered this only as a result of doing a few experiments in response to a question on the OTN database forum.

Here’s a little demonstration, cut-n-pasted with a little cosmetic editing from an 11gR1 SQL*Plus session: