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Column Groups

I think column groups can be amazingly useful in helping the optimizer to generate good execution plans because of the way they supply better details about cardinality; unfortunately we’ve already seen a few cases (don’t forget to check the updates and comments) where the feature is disabled, and another example of this appeared on OTN very recently.

Modifying the example from OTN to make a more convincing demonstration of the issue, here’s some SQL to prepare a demonstration:


Here’s a live one from OTN – here are a couple of extracts from the problem statement:

We’re experiencing an issue where it seems that the query plan changes from day to day for a particular procedure that runs once a night.
It’s resulting in a performance variance of 10 second completion time vs 20 minutes (nothing in between).
It started occurring about 2 months ago and now it’s becoming more prevalent where the bad query plan is coming up more often.
I noticed that the query plans vary for a simple query.
We do run gather statistics every night. (DBMS_STATS.GATHER_SCHEMA_STATS (ownname=>sys_context( ‘userenv’, ‘current_schema’ ), estimate_percent => 1);)

The query and two execution plans look like this:

Histogram Tip

I’ve just responded to the call for items for the “IOUG Quick Tips” booklet for 2015 – so it’s probably about time to post the quick tip that I put into the 2014 issue. It’s probably nothing new to most readers of the blog, but sometimes an old thing presented in a new way offers fresh insights or better comprehension.

Histogram Tips

A histogram, created in the right way, at the right time, and supported by the correct client-side code, can be a huge benefit to the optimizer; but if you don’t create and use them wisely they can easily become a source of inconsistent performance, and the automatic statistics gathering can introduce an undesirable overhead during the overnight batch. This note explains how you can create histograms very cheaply on the few columns where they are most likely to have a beneficial effect.


Here’s a simple data set – I’m only interested in three of the columns in the work that follows, but it’s a data set that I use for a number of different models:

Index Usage – 4

Here’s a thought that came to me while I was writing up a note about identifying redundant indexes a few minutes ago. Sometimes you end up supporting applications with unexpected duplication of data and indexes and need to find ways to reduce overheads. Here’s some code modelling a scenario that I’ve seen more often than I like (actually, just once would be more often than I’d like):

[Oracle] Insights into SQL hints - Embedded global and local hints and how to use them


The idea for this blog post started a few weeks ago when i had to troubleshoot some Oracle database / SQL performance issues at client site. The SQL itself included several views and so placing hints (for testing purpose) into the views was not possible, especially as the views were used widely and not only by the SQL with the performance issue. In consequence this blog post is about the difference between embedded global and local hints and how to use them.


CBO catchup

It’s interesting to watch the CBO evolving and see how an enhancement in one piece of code doesn’t necessarily echo through to all the other places it seems to fit. Here’s an example of an enhancement that spoiled (or, rather, made slightly more complicated) a little demonstration I had been running for about the last 15  years  – but (in a fashion akin to another partitioning limitation) doesn’t always work in exactly the way you might expect.

Subquery Factoring (10)

What prompted me to write my previous note about subquerying was an upgrade to 12c, and a check that a few critical queries would not do something nasty on the upgrade. As ever it’s always interesting how many little oddities you can discover while looking closely as some little detail of how the optimizer works. Here’s an oddity that came up in the course of my playing around investigation in – first some sample data:

Subquery Factoring (9)

Several years go (eight to be precise) I wrote a note suggesting that Oracle will not materialize a factored subquery unless it is used at least twice in the main query. I based this conclusion on a logical argument about the cost of creating and using a factored subquery and, at the time, I left it at that. A couple of years ago I came across an example where even with two uses of a factored subquery Oracle still didn’t materialize even though the cost of doing so would reduce the cost of the query – but I never got around to writing up the example, so here it is:

12c Downgrade

No, not really – but sometimes the optimizer gets better and gives you worse performance as a side effect when you upgrade. Here’s an example where recognised (with a few hints) the case for a nested loop semi-join and 12c went a bit further and recognised the opportunity for doing a cunning “semi_to_inner” transformation … which just happened to do more work than the 11g plan.

Here’s a data set to get things going, I’ve got “parent” and “child” tables, but in this particular demonstration I won’t be invoking referential integrity: