Hints

davidkurtz's picture

PeopleTools 8.54: %SQLHint Meta-SQL

This is part of a series of articles about new features and differences in PeopleTools 8.54 that will be of interest to the Oracle DBA.
 
This new PeopleCode meta-SQL macro performs a search of SQL statement for the nth instance of SQL command keyword and inserts a string after it.

davidkurtz's picture

PeopleTools 8.54: %SQLHint Meta-SQL

This is part of a series of articles about new features and differences in PeopleTools 8.54 that will be of interest to the Oracle DBA.
 
This new PeopleCode meta-SQL macro performs a search of SQL statement for the nth instance of SQL command keyword and inserts a string after it.

Jonathan Lewis's picture

push_pred – evolution

Here’s a query (with a few hints to control how I want Oracle to run it) that demonstrates the difficulty of trying to solve problems by hinting (and the need to make sure you know where all your hinted code is):

Jonathan Lewis's picture

Baselines

I’m not very keen on bending the rules on production systems, I’d prefer to do things that look as if they could have happened in a completely legal fashion, but sometimes it’s necessary to abuse the system and here’s an example to demonstrate the point. I’ve got a simple SQL statement consisting of nothing more than an eight table join where the optimizer (on the various versions I’ve tested, including 12c) examines 5,040 join orders (even though _optimizer_max_permutations is set to the default of 2,000 – and that might come as a little surprise if you thought you knew what that parameter was supposed to do):

Jonathan Lewis's picture

First Rows

Following on from the short note I published about the first_rows optimizer mode yesterday here’s a note that I wrote on the topic more than 2 years ago but somehow forgot to publish.

I can get quite gloomy when I read some of the material that gets published about Oracle; not so much because it’s misleading or wrong, but because it’s clearly been written without any real effort being made to check whether it’s true. For example, a couple of days ago [ed: actually some time around May 2012] I came across an article about optimisation in 11g that seemed to be claiming that first_rows optimisation somehow “defaulted” to first_rows(1) , or first_rows_1, optimisation if you didn’t supply a final integer value.

davidkurtz's picture

To Hint or not to hint (Application Engine), that is the question

Over the years Oracle has provided a number of plan stability technologies to control how SQL statements are executed.  At the risk of over simplification, Outlines (deprecated in 11g), Profiles, Baselines and Patches work by injecting a set of hints into a SQL statement at parse time.  There is quite a lot of advice from Oracle to use these technologies to fix errant execution plans rather than hint the application.  I think it is generally good advice, however, there are times when this approach does not work well with PeopleSoft, and that is due to the behaviour and structure of PeopleSoft rather than the Oracle database.
davidkurtz's picture

To Hint or not to hint (Application Engine), that is the question

Over the years Oracle has provided a number of plan stability technologies to control how SQL statements are executed.  At the risk of over simplification, Outlines (deprecated in 11g), Profiles, Baselines and Patches work by injecting a set of hints into a SQL statement at parse time.  There is quite a lot of advice from Oracle to use these technologies to fix errant execution plans rather than hint the application.  I think it is generally good advice, however, there are times when this approach does not work well with PeopleSoft, and that is due to the behaviour and structure of PeopleSoft rather than the Oracle database.
Jonathan Lewis's picture

SQL Plan Baselines

Here’s a thread from Oracle-L that reminded of an important reason why you still have to hint SQL sometimes (rather than following the mantra “if you can hint it, baseline it”).

I have a query that takes 77 seconds to optimize (it’s not a production query, fortunately, but one I engineered to make a point). I can enable sql plan baseline capture and create a baseline for it, and given the nature of the query I can be confident that the resulting plan will always be exactly the plan I want. If I have to re-optimize the query at any time  (because it runs once per hour, say, and is constantly being flushed from the library cache) how much time will the SQL plan baseline save for me ?

The answer is NONE.

The first thing that the optimizer does for a query with a stored sql plan baseline is to optimize it as if the baseline did not exist.

Jonathan Lewis's picture

Delete Costs

One of the quirky little anomalies of the optimizer is that it’s not allowed to select rows from a table after doing an index fast full scan (index_ffs) even if it is obviously the most efficient (or, perhaps, least inefficient) strategy. For example:

Jonathan Lewis's picture

Delete Costs

One of the quirky little anomalies of the optimizer is that it’s not allowed to select rows from a table after doing an index fast full scan (index_ffs) even if it is obviously the most efficient (or, perhaps, least inefficient) strategy. For example:

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