Thursday, June 21, 2012

Can You Take a Hint?

Hints in Oracle have been around since version 8. Hints are like compiler directives that tell Oracle what path to take when optimizing a SQL statement (generally speaking.) However, Oracle will ignore the hint if it can’t do it or it is formatted poorly.

Most tuning products for SQL will make use of hints if statement re-arrangement doesn’t solve the problem. In later versions of Oracle, outlines became available. You see in many applications the SQL is considered source code and the end user is of course not allowed to modify them. If you cannot place the hint into the code then of course hints could not be used for those applications with source code restrictions. Outlines allowed hints to be tied to a SQL statement ID tag (a signature) and to be applied at run time by creating a execution plan containing the hints and storing that plan to be used when the SQL with the proper ID was recognized by the Oracle kernel. Profiles were the first stage of execution plan stabilization (ensuring that for a specific SQL statement the execution plan stayed the same, even with changes in the underlying statistics at the table and index level.)

Now in 10g and beyond Oracle provides not only hints and outlines, but a new execution plan stabilization mechanism called profiles. Profiles alter the costs of various optimizer branches to optimize a specific piece of SQL code.

Generally speaking adding hints was considered to be the last thing you did if you couldn’t get the SQL optimizer to properly optimize the SQL statement through the use of indexes, statement rewrite or initialization parameter adjustments. Why? Well, by adding hints to SQL statements you generated possible documentation issues if you didn’t properly document where hints had been placed in the application code and why. The use of hints also could result in excessively stable plans that wouldn’t change even if they needed to, not a good situation to be in. Finally hints are changed, dropped and deprecated in each release of Oracle, so with each patch or upgrade you would need to re-verify each hinted statement to be sure Oracle didn’t send along a little surprise in the upgrade or patch.

 In my opinion it is a good practice to only use hints when no other method of SQL statement optimization can be applied. This also applies to outlines and profiles if you don’t have access to change source code.   

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