The optimizer generates the execution plan based on the presence of the bind. Motivation behind cbo is to come up with the cheapest execution plan available for each sql statement. I cannot observe any differences in execution plans with or without the hint and on my system both run with rbo. A value of 10 means that the cost of an index access path is onetenth the normal cost of. Query executes in seconds with the rule hint and several. Oracle 12c optimizer enhancements eye on databases. It is only present to provide backwards compatibility during the migration to the query optimizer cost based optimizer. The following considerations apply to both the cost based and rule based approaches. There is no such thing as a rule based query well, you can use the rule hint but thats really not the same. Based on this information, you might be able to choose a more efficient execution plan. Hints, optimizer directives can be a better name as jonathan lewis advises, can be very dangerous sometimes. Earlier, the only optimizer in the oracle database was the rule based optimizer rbo. Yes, the rulebased optimizer still lives in oracle database. They may behave differently release to release, they do not produce.
Ask tom cost rule based optimizer behaviour oracle. With a large number of oracle shops using the rule based optimizer rbo, migration to cost based optimization cbo will become a important task when migrating to oracle10i, where the rbo will disappear. If an index was available on a table, the rbo rules said to always use the index. New oracle10g sql optimizer hints by donald burleson in data management on april 26, 2004, 12. Oracle 12c elevates the behavior of optimizer at the instrumental level by. All applications should be converted to use the cost based optimizer as the rule based optimizer is not supported in oracle 10g and above releases. Rule hint still popular in oracle 10g for many years since the release of oracle8, oracle has threatened to phaseout the rule based optimizer rbo. Yes, the rulebased optimizer still lives in oracle.
The optimizer recognizes such situations based on unique and primary key constraints on the tables. The rulebased optimization is available for backward compatibility with. Hints give us the opportunity, in cases where we have superior knowledge about the database, to influence the optimizer. Without the hint, the optimizer could make a query optimization decision to use the best plan generated without the transformation, instead of the best plan for the transformed query. This hint explicitly chooses the costbased approach to optimize a sql. Jul 22, 20 the rulebased optimizer is no longer supported in oracle database 10g release 1 10. Oracle optimizer hints oracle optimizer hints when using. If an index scan is available, the optimizer may choose it over a full table scan. The rbo was in use for many years, and lots of applications were developed using the rule based optimizer.
Oracle decided to dismiss the rule based optimizer from version 10g, leaving the cost based one as the only choice. Not that i dont trust him, but i had the impression that cost based was the way to go. Earlier, the only optimizer in the oracle database was the rulebased optimizer rbo. Nov 20, 2001 there is no such thing as a rule based query well, you can use the rule hint but thats really not the same. I was in the latter scenario, as my first oracle 11g database environment required not just new data, but a new database weekly and a requirement for me to guarantee. Oracle database 11g new features adaptive cursor sharing. The rulebased optimizer rbo will no longer be supported when oracle9i is desupported. As time passed from oracle 9i to 10g, we harnessed hints less, trusting the cbo and by oracle 11g, it started to be frowned upon unless you had a very strong use case for hinting. We do not provide patches or new release for versions or products that are retired, discontinued or cloud based solutions. What drew me to investigate the rule based optimizer.
This hint causes the optimizer to make these choices. The correct selectivity and cost is critical for the oracle optimizer to allow it to make the correct decision in selecting an execution plan when parsing an. Oracle toad expert blog for developers, admins and data analysts. The rbo was in use for many years, and lots of applications were developed using the rulebased optimizer. Remote dba oracle10g optimizer rule costbased optimizer 10g. With a large number of oracle shops using the rulebased optimizer rbo, migration to costbased optimization cbo will become a important task when migrating to oracle10i, where the. I know what its declared in oracle 10g rule hint is not supported and rbo is not supported, but rule hint. I asked him why and he said that the siebel application defaults to rule based optimization. With solutions for toad for oracle, toad for mysql, toad for sql server, db2, sap and more. Up until the release of oracle8i, the rule based optimizer often generated a faster execution plan than cost based optimization. It will rarely choose to use an index on column referenced using an or statement.
Avoid using an index that fetches many rows from the driving table when you can use. Upgrading from 9i to 11g and the implicit migration from rbo. The rule based optimizer has not been updated in nearly a decade and is not recommended for production use because the rbo does not support any new features of oracle since 1994 such as bitmap indexes, table. This means the optimizer will no longer operate under rule mode rbo when a table has no statistics. The oracle database 11g reference makes no mention of the venerable rule and choose options and the demise of the rule based optimizer was therefore treated as fact. For oracle applications, it was ironic that oracle apps used the rbo, and oracle made a big deal out of announcing that oracle applications 11i ebusiness suite was moving to the cost based sql. View notes oracle optimizer hints from dbst 651 at university of maryland, university college. However, a simple experiment will prove that the rule based optimizer is alive and well in oracle database 11g. The following object query properties force the cbo to be used even when the rule.
True or false optimizer mode and tuning goals charles. When i changed the query to run on rule based optimizer, i go the result in say less than 30 sec. Oracle10g will only support the costbased optimizer cbo. There is a hint in the cost based optimizer to have this happen as well. The driving table is the table we will join from that is join to other tables. Oracle magazine subscriptions and oracle white papers. Optimizer defaults are adequate for most operations, but not all. The rule mode is ignored if the statement contains hints other than the rule hint itself, hence their cost based plan. Optimizer mode hints are used to redirect the overall optimizer goal. The optimizer first determines whether joining two or more of the tables definitely results in a row source containing at most one row. The functionality is still present but no new functionality has been included in it and it is no longer supported by oracle. Oracle rule based optimizer rbo prior to version 10.
Sep 21, 2011 i was reminded of this story on reading an otn discussion on whether the rule based optimizer is still available in oracle database 11g. Oracle, rulebased optimization,rbo costbased optimization,cbo hint. Hence oracle9i release 2 is the last release to support the rbo. Apr 26, 2004 new oracle10g sql optimizer hints by donald burleson in data management on april 26, 2004, 12. Is it possible to set up a database schema so that it the optimizer always uses rule based rather than cost. The optimizer functions with objects statistics, selectivity and cardinality to calculate the cost of data fetch operation and ultimately, exposing the best execution plan. However, if you are currently on oracle 9i then you will have to tackle both the upgrade to a new release and the migration from the rule based optimizer rbo to the cost based optimizer cbo. You can use optimizer hints with sql statements to alter execution plans. This section describes new performance features of oracle database 10g release 1. Oracle decided to dismiss the rulebased optimizer from version 10g, leaving the costbased one as the only choice.
Please give me all the negative impacts of this hint in 10g. The rulebased optimizer has not been updated in nearly a decade and is not recommended for production use because the rbo does not support any new features of oracle since 1994 such as bitmap indexes, table. As with oracle 10g, there is only cbo, there is no imperative need to set this parameter to use the cbo. Basically, the rbo used a set of rules to determine how to execute a query. The rulebased optimizer is no longer supported in oracle database 10g release 1 10. However, it will influence the the plan either to have a the first rows as soon as possible. Ask tom rule based optimizer used consider using cbo. Dec 19, 2011 the rule mode is ignored if the statement contains hints other than the rule hint itself, hence their cost based plan. This hint can be used with either rule based or cost based optimization. Can anyone tell me if there is still a rule based optimiser in 10g, i have heard that it has been removed. It should be noted that the optimizer hints only cause oracle to apply the optimizer to the sql, and additional hints can still be added for further tuning.
Switching from rulebased optimizer to costbased oracle. This is the cost based approach designed to provide the best response time. The user wants the first 10 employees of department 20 to be displayed as. Optimizer is the key driver component in the execution of sql query in oracle database. On one hand, the documentation goes out of its way to tell you to use them sparingly, but on the other hand, you can choose from more than 60 hints in 10g and more than 70 in 11g 64 and 71, to be precise. Oracle optimizer hints when using cost based optimization, it is possible to. Index skip scanning in oracle 9i and above allow indexes to be used even when the leading edge is not referenced. The end of rule based optimization in oracle 10g by scott stephens in data centers on may 24, 2004, 9. Jun 27, 2014 optimizer is the key driver component in the execution of sql query in oracle database. Free oracle magazine subscriptions and oracle white papers. The rule based optimizer rbo is now obsolete in oracle 10g. My doubt is will this hint create any problem if my db is upgraded to 10g. An optimizer hint is a code snippet within an sql statement controlling the decisions of the optimizer.
There is a method to automate the gathering of stats only for objects that need it. The parameter accepts one of a list of valid string values corresponding to the release numbers, such as 11. The oracle hint list is inside the oracle executable and you can extract the oracle hint list easily with unix commands. The most common mode for sql tuning is the rule hint, which is used when you suspect that the cbo is using a suboptimal execution plan. Dec 21, 2009 now that oracle database 11g release 2 is out, more and more folks are considering upgrading to 11g. However, if you are currently on oracle 9i then you will have to tackle both the upgrade to a new release and the migration from the rulebased optimizer rbo to the costbased optimizer cbo. The end of rulebased optimization in oracle 10g by scott stephens in data centers on may 24, 2004, 9. The cheapest plan is the one that will use the least amount of resources cpu, memory, io, etc. There are some cases where the use of an index slowed down a query. Optimizer mode rule or choose 3520 mar 28, 2007 10. So oracles implementation of it is two features, adaptive cursor sharing and cardinality feedback, and of course its different in many ways from the other products on the market, but the concept that the database vendors are getting into is, well, lets take some of the runtime data and send it back to the optimizer. Anyone using oracle 10g should be sure to use the 10. Although the rbo was depreciated in oracle database 10g, the rule mode and hint are still honored assuming the query can actually use the rbo. Therefore, even though you gave the index hint, the optimizer might not.
Optimizer hints are an interesting feature of oracle. Ive seen instead oracle 10g changing execution plans from night to day, leading to turtlelike performances. Rule hint still popular in oracle 10g for many years since the release of oracle8, oracle has threatened to phaseout the rulebased optimizer rbo. A value of 10 means that the cost of an index access path is onetenth the normal cost of an index access path.
An effective approach for query tuning involves enabling the rule based optimizer either through a rule hint, or by changing the sessionlevel optimizer goal with the following sql statement. Now that oracle database 11g release 2 is out, more and more folks are considering upgrading to 11g. The rule hint ignores the table and index statistics and uses basic heuristics to generate an execution plan. When using the rule based optimizer the placement of tables in the from clause is relevant. You can use grep and strings to get them directly from the oracle executable. The use of or statements confuses older versions of the cost based optimizer cbo. If not analyzed, you can set table, index for analyze. The oracle database 11 g reference makes no mention of the venerable rule and choose options and the demise of the rule based optimizer was therefore treated as fact. Please view the product lifecycle table found on the product support page to determine support status for this product. The application designer can use hints in sql statements to specify how the statement should be executed. For oracle applications, it was ironic that oracle apps used the rbo, and oracle made a big deal out of announcing that oracle applications 11i ebusiness suite was moving to the costbased sql.
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