Oracle Good Hints List

Question:  Could you share your good hints list please?  Some Oracle people say that there are "good" and bad" hints, but I don't agree that some hints like the ordered hint belong on the bad hint list.

Answer:  If the Oracle optimizer always made correct guesses about the size of internal result sets, hints would be far less common, but make no mistake, it's impossible for any software product to always make the correct guess, based on the limited information in the data dictionary.

I have the ordered hint on my good hint list because there are times when I know better than the optimizer about the future state of the tables, and in cases where the is one, and only one optimal table join order, the ordered hint is a good hint because it can save the optimizer a huge amount of processing at parse time, especially for SQL that joins more than 10 tables.

Good hint or bad hint, hints as a last-resort and your should always try to find the rot cause of your suboptimal plan (stale/missing statistics, wrong optimizer_mode), etc.

When the optimizer makes a mistake, it's very tempting to add a "good" hint, and move the SQL into production.  In practice, while your sub-optimal SQL is not a huge issue, it is a symptom of a much larger issue, and fixing your execution plan may also have the side effect of improving the execution plans of thousands of other SQL statements!

There are "good" hints, those that help the optimizer make a better decision. 

The global optimizer mode hints (all_rows, first_rows_1, first_rows_10, first_rows_100, first_rows_1000) are good hints because they are used when the majority of the SQL workload is optimal, but there are "outlier" SQL that needs a different optimizer mode.

The only exceptions are hints that give "a priori" information to the CBO like these good hints:

Don Burleson good hint list:

• append
• (no)append
• parallel
• (no)parallel
• rewrite
• (no)rewrite
• all_rows
• cardinality
• cpu_costing
• cursor_sharing_exact
• driving_site
• dynamic_sampling
• first_rows_1
• first_rows_10
• first_rows_100
• first_rows_1000
• opt_param
• ordered
• ordered_predicates
• selectivity
• skip_ext_optimizer

Of course, you could argue that the CBO would not need some good hints (e.g. number of rows returned by query, table join order, etc.), but sometimes there is no substitute for a-priori knowledge.

 

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