From time to time I check the site statistics to see if they give me any clues about why people are coming to blog – and recently I noticed that over the last year a particular referral from the OTN Database forum was had appeared fairly regularly – and it’s one that covers a small but significant optimizer detail that combines two crtical questions: why is the optimizer not using my index and why is the optimizer ignoring my hint under the heading “Index hint does not work”.
I’ll leave you to read the thread – but the short answer is NULL.
A hint is illegal if using it could produce the wrong answer, and indexes where every column is nullable won’t necessarily reference every row in its table.
[Further reading on “ignoring hints”]
When I am in America for a few days I usually try to find a local user group and ask them if they’d like to arrange an evening slot for their members. (Last time I was at Oracle Open World I went so far as to do a day trip up to Calgary because I didn’t think I was likely to go there any other way).
Collaborate 09 was no exception, and I contacted the CFOUG who arranged an event on the Monday evening at which I did a presentation called “Optimising through Understanding” where I talk about the analysis that could go into one simple SQL statement, producing reasons why any of several different execution plans might be the most appropriate depending on circumstances.
The presentation is a variation of one I have now given several times – and there is even a voice-over webinar version of it that I linked to from an earlier blog item.
After the event James Taylor, the chairman of CFOUG, asked if I could let him have a copy of the presentation for their website – so I sent him a pdf file of the slides, and I’ve also posted the same pdf file on my blog.
This is the title of a presentation I have given a few times in the last couple of years – most recently at Collaborate 09 in Orlando.
The IOUG has now posted on their website the presentation and a short article I wrote to go with it; but that’s for members only, so it seemed reasonable to publish the same thing on my own blog as well.
So here are links for a pdf file of the presentation, and the article in Word format.
I try to avoid hinting SQL if possible as it is very hard to do well, but there are a few hints that give an overview of how a query should operate without trying to control the detail of what the optimizer does. These are the hints that I call the “strategic hints” (possibly “query block hints” would be a better name – but there may be hints at the query block level that I wouldn’t qualify as strategic), and an example came up on the comp.databases.oracle.server newsgroup recently that looks like an ideal example of how a couple of them could be used.
Someone recently sent me a request about a piece of SQL they could not optimise. I don’t usually respond to private requests – it’s not an effective use of my time – but their example was something that pops up relatively frequently as a “bug” – so I thought I’d mention it here.
The SQL looked like this:
insert into tab3
select -- small result set
tab1@dblink t1 -- large data set
tab1.col1 in (
tab2 -- small data set
When it comes to setting the optimizer_mode parameter you often hear people say that first_rows_N (for one of the legal values of N) should be used for OLTP systems and all_rows should be used for decision support and data warehouse systems.
There is an element of truth in the statement – but it’s really a hangover from the early days of CBO, and remembrance of the old first_rows optimizer mode (** See footnote).
[Back to Manual Optimisation part 2]
This little series started from a note I wrote about manual optimisation where I took advantage of a sort operation in a non-mergeable view to produce sorted data from a final nested loop join without including an “order by” that would have produced a large sort operation.
In fact, as I showed in a follow-up post, this was taking a convenient pagination mechanism to an extreme – and you might decide (with good reason, as Tom Kyte did) that it was an extreme that should not be used.
I’ve written a number of notes about hinting in fact, by using at the “Select Category” list to the right, I see that I have (so far) tagged 26 different articles (and this will be the 27th) with the hints tag. So I’ve decided it was time that I made clear my basic guidelines on safe hinting, as follows:
- If you must use hints, then assume you’ve used them incorrectly.
- On every patch or upgrade to Oracle, assume every piece of hinted SQL is going to do the wrong thing … because of (2) above. You’ve been lucky so far, but the patch/upgrade lets you discover your mistake.
- Every time you apply some DDL to an object that appears in a piece of hinted SQL assume that the hinted SQL is going to do the wrong thing … because of (2) above. You’ve been lucky so far, but the structural change lets you discover your mistake.
You will appreciate from these guidelines that I don’t really approve of using hints. The only reason that I leave them in place on a production system is when I’m sure that there is no alternative safe mechanism for making the optimizer do what I want. (And that does mean that I will use hints sometimes on a production system.)
What I use them for on test systems is to check whether a particular execution plan is actually possible, and to track down bugs in the optimizer.
Finally, for the purposes of education, I use them to demonstrate execution plans without first having to craft data sets and set database parameters to make a plan appear ‘spontaneously’.
Always be cautious about adding hints to production systems.
From time to time I’ve commented on the fact that setting cursor_sharing to force or similar may be a temporary workaround to bad coding practises, but that it can introduce problems, has a few associated bugs, and shouldn’t be viewed as a guaranteed, or long-term, solution.
Some time ago I wrote a note (on my website) about the push_pred() and no_push_pred() hints. I’ve recently discovered a bug in the 9.2 optimizer that means you may find that Oracle will not use “join predicate pushdown (JPPD)” when it is obviously a good idea.
This note discusses a sequence of execution plans, taken from a system running 184.108.40.206, to demonstrate the point.
I’ve written about subquery factoring a few times in the past and commented on the use of the /*+ materialize */ hint. Recently I had time to think about what it would take for the Cost Based Optimizer to decide to materialize a subquery without hinting.
I doubt if I have a complete answer yet, and I sometimes wonder if the optimizer code for handling subquery factoring is not yet complete, but my observations are as follows.
The following question appeared on the Oracle Forums recently:
The use of functions – a function with other selects (eg. calculate availability of a part) – is slowing down our system when we do a select over our product file.
Is there some kind of rule when functions should be used or when we should try to create a more complex – combined – SQL that does not use the function.
Can functions be used in the where clause without loosing a lot of speed?
It’s a really good question, because it prompts such a lot of ideas that need to be tied together, so I thought I’d jot down a few thoughts.
When Oracle tries to optimize a complex SQL statement one of the first steps it takes is to transform it into something simpler. The ideal target (from the optimizer’s perspective) is to eliminate any subqueries and in-line views so that your SQL statement ends up as a list of tables in a single from clause. Every in-line view and subquery is a separate query block, and the optimizer really wants to turn the whole thing into a single query block. Unfortunately this isn’t always possible, and sometimes it isn’t even a good idea – which is why 10g introduced ‘cost base query transformation’.
How well do you know your hints? When you see this one /*+ parallel(t1 4) */ what, exactly, does it mean ? The answer may not be what you think – and if you don’t know exactly what a hint means, how can you make it “work properly” ?
Another little warning for those about to upgrade from 9i to 10g. Some hints will change their behaviour. Consider the following SQL.