Why is Oracle ignoring my hints ?
I have a table and want to count the rows, so here’s the query and execution plan I get on the first attempt:
select /*+ full(t) */ count(*) from t1 t;
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
| 0 | SELECT STATEMENT | | 1 | 79 (2)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | INDEX FAST FULL SCAN| T1_N2 | 47343 | 79 (2)| 00:00:01 |
A few months ago, I wrote a note about setting the optimizer_mode to one of the first_rows_N values (first_rows_1, first_rows_10, first_rows_100, or first_rows_1000).
One of the effects associated with this parameter is that the first_rows(N) hint and the predicate “rownum <= N” use the same first_rows_N arithmetic (although N can take any value for the rownum or hint).
In a recent follow-up, Timur Akhmadeev supplied a link to a discussion on the Oracle Forum about this topic, starting with a problem that a rownum predicate was causing and ending with a resolution through the row_number() analytic function.
I thought it would be worth making it easier for future researches to find the discussion by creating a specific blog item to point to it.
Some time ago I wrote a note about distributed DML, pointing out that the driving_site() hint works with distributed queries but not with DML based on distributed queries; so insert as select, or create as select and so on will “ignore” the hint.
This is just a little follow-up to give you an idea of what execution plans for distributed queries look like so that you can tell whether your query is going to work locally or remotely.
The bits of Oracle which aren’t documented always seem to be the bits that are hard to resist, so I thought I’d make a brief comment on undocumented hints.
Of course, you should not take advantage of any undocumented feature without first getting approval from Oracle support, but some hints seem to me to fall into a special category where you are more likely to get that approval – and here are my thoughts on why.
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 126.96.36.199, 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.