Five reasons why I avoid database query hintsJul 28, 2010 / Posted By:Robert Buda
I know that some will disagree with me on this but I have never been a fan of database query hints.
Most database vendors give you the ability to override the query optimizer with hints that you specify in a query. Each query can be executed in a variety of ways by the database engine. Normally, the optimizer evaluates the statistics about the data in each of the tables involved in the query and makes a decision about the best way to execute that query. Hints enable you to direct the database engine to use a specific approach, even if it prefers a different one.
Here are five reasons that I avoid hints.
Reason Number 1. I view hints as a workaround to a problem that should be solved in other ways. There are a few problems that may lead to the need to use a hint:
- One is that the database statistics may not be up to date which prevents the optimizer from making the best choices. The obvious solution to this problem is to ensure that all appropriate statistics are collected frequently.
- Another problem may be with the database or application design. If the query optimizer cannot choose a path that will execute the query quickly enough, we may need to look at the use of indexes or even the data model itself. Does the data model actually represent the true relationship between data or is the model causing the application to do more work than it should?
- Finally, all major database vendors upgrade and improve their optimizers over time. For example, Oracle’s optimizer has improved very significantly over the past few major releases. If you are not on a recent release of you database software, Upgrade! When you do, rip out your hints and see how the new optimizer does, you may be surprised.
Reason Number 2: Database vendors change their optimizers with new releases. Hints that have good results in one release can have poor results or even cause a query to break in later releases.
Reason Number 3: As data volumes grow and statistics are updated in the database, hints prevent the optimizer from selecting new and better execution paths as the result of the change in the characteristics of the data.
Reason number 4: As much as we would like to think that we are smarter than Microsoft or Oracle, most of us have not put millions of dollars and many years into query optimization technology as they have. Since we are paying for this through our license fees, we should take advantage of that investment.
Reason number 5: Now the reason that led me to write this post: This excellent blog post describes situations where a hint can cause a query to succeed when one set of criteria is given and to fail when other criteria is given. This type of problem is very difficult to diagnose because it is sporadic and depends on specific criteria. This problem occurs on SQL Server and in that environment, this issue alone is enough to rule out the use of query hints.
Bottom line: use query hints only as a last resort. Getting to and fixing the root cause of the problem will ensure that the query will continue to perform well into the future.