SQL Plan Directives: they’re always good… except when they’re bad!

The new Oracle 12c optimizer adaptive features are just great and work well out of the box in most cases.

Recently, however,  I’ve experienced my very first problem with SQL Plan Directives migrating a database to 12c, so I would like to share it.

Disclaimer 1: this is a specific problem that I found on ONE system. My solution may not fit with your environment, don’t use it if you are not sure about what you’re doing!

Disclaimer 2: despite I had this problem with a single SPD, I like adaptive features and I encourage to use them!!

Problem: a query takes a sub-second in 11gR2, in 12c it takes 12 seconds or more.

V_TAB_PROP is a very simple view. It just selects a central table “TAB” and then takes different properties by joining  a property table “TAB_PROP”.

To do that, it does 11 joins on the same property table.

On the property table, TAB_PROP_ID and PROP_ID are unique (they compose the pk), so nested loops and index unique scans are the best way to get this data.
The table is 1500Mb big and the index 1000Mb.

This was the plan in 11g:

In 12c, the plan switches to adaptive, and half of the joins are converted to hash joins / full table scans:

However, the inflection point is never reached. The execution keeps the default plan that has half of the joins HJ and the other half NL.

The problem in this case is the SQL Directive. Why?

There are to many distinct values for TAB_ID and the data is very skewed.

The histogram on that column is OK and it always leads to the correct plan (with the adaptive features disabled).
But there are still some “minor” misestimates, and the optimizer sometimes decides to create a SQL Plan directive:

The Directive instructs the optimizer to do a dynamic sampling, but with a such big and skewed table this is not ok, so the Dynamic sampling result is worse than using the histogram. I can check it by simplifying the query to just one join:

What’s the fix?

I’ve tried to drop the directive first, but it reappears as soon as there are new misestimates.
The best solution in my case has been to disable the directive, an operation that can be done easily with the DBMS_SPD package:

I did this on a QAS environment.
Because the production system is not migrated to 12c yet, it’s wise to import these disabled directives in production before the optimizer creates and enables them.

Off course, the directives can’t be created for objects that do not exist, the import  has to be done after the objects migrate to the 12c version.

Because the SQL Plan Directives are tied to specific objects and not specific queries, they can fix many statements at once, but in case like this one, they can compromise several statements!

Monitoring the creation of new directives is an important task as it may indicate misestimates/lack of statistics on one side or execution plan changes on the other one.

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Ludovico

Oracle ACE and Senior Consultant at Trivadis SA
Ludovico is an Oracle ACE Director, frequent speaker and community contributor, working as Senior Database Specialist for Trivadis, Switzerland.

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3 thoughts on “SQL Plan Directives: they’re always good… except when they’re bad!

  1. Pingback: SQL Plan Directives: they’re always good… except when they’re bad! - Ludovico Caldara - Blogs - triBLOG

  2. Pingback: 12c: директивы плана выполнения — SQL Plan Directives | Oracle mechanics

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