The purp ose of this c hapter is to primarily discuss the core problems in query optimization and their solutions, and only touc. The area of query optimization is v ery large within the database eld. Optimization is performed in the third stage of the compilation process. Cost based query optimization in distributed databases cost based qo.
The oracle cost based optimizer is designed to determine the most effective way to carry out the sql statement. Using this approach, the transformed query is compared to the original query, and oracles optimizer then selects the best execution strategy. Query optimization is the part of the query process in which the database system compares different query strategies and chooses the one with the least expected cost. The cheapest plan is the one that will use the least amount of resources cpu, memory, io, etc.
If you want to understand how oracles cost based optimizer works, you will want to read this book. The oracle optimizer and adwc statisticsbased query transformation. Time is not the parameter based on which you decide. Motivation behind cbo is to come up with the cheapest execution plan available for each sql statement. What are the query optimization techniques in oraclesql. Timesten and timesten cache have a costbased query optimizer that ensures efficient data access by automatically searching for the best way to answer queries. Oracle introduced the costbased optimizer cbo over a decade ago, with the.
Other optimization models have been proposed, especially in the areas of parallel query optimization, using cost models that arenot cardinalitybased but instead deal with resource scheduling and allocation 7. Also if you have a compound index x1 column should be ordered first in the index in order for the index to be used. As you migrate to oracle10g, you may need to undertake a migration from rbo to cbo. Sql optimization interview questions performance tuning. Costbased oracle fundamentals jonathan lewis apress. In a costbased optimization strategy, multiple execution plans are generated for a given query, and then an estimated cost is computed for each plan. The optimizer estimates the cost of each plan based on statistics in the data dictionary.
This paper describes costbased query transformation in oracle relational database system, which is a novel phase in query optimization. For this reason, the optimizer is sometimes called the costbased optimizer cbo to. To execute the query, oracle performs a range scan of. The execution plan selected for a sql statement is just one of the many alternative execution plans considered by the optimizer. In addition, nonstandard query optimization issues such as higher level query evaluation, query optimization in distributed databases, and use of database machines are addressed. In this blog i explained basics of costbased optimization and how its works. If an index was available on a table, the rbo rules said to always use the index. Packaged alongside the 195e release and higher, the cbos primary job is to explore the space of possible query plans and to find the most optimal.
In this post, im going to cover a clever optimization included in adwc. If one table involved in a statement does not have statistics, oracle has to revert to rulebased optimization for that statement. Oracles costbased sql optimizer cbo is an extremely sophisticated component of oracle that governs the execution for every oracle query. This spj expression is run through a costbased optimizer that seeks to find the most efficient combination of physical operators to produce the. Your answer needs a long answer but i will try to give as short answer as possible. First up, you need a way to measure performance of your query. The costbased optimizer is simply a piece of code that contains a model of how oracle databases work. Using this information, the query optimizer is able to compute plan costs with a high degree of accuracy and choose the best execution plan based on the least cost. Adaptive query optimization in oracle database 12c 12. It decides the query execution plan by deciding the statistical information based on the database objects.
This is part 2 of a series on the oracle optimizer in the oracle autonomous data warehouse cloud. If the flag is off, the optimizer chooses subquery materialization over in to exists subquery transformation. The query transformer employs several query transformation techniques. There are some cases where the use of an index slowed down a query.
The query optimizer uses these two techniques to determine which process or expression to consider for evaluating the query. The query optimizer attempts to determine the most efficient way to execute a given query by considering the possible query plans generally, the query optimizer cannot be accessed directly by users. The cost number is the estimated number of physical io operations oracle thinks it will have to find the requested data, based solely on statistics. It presents the framework for costbased query transformation. The sql server query optimizer is based on cost, meaning that it decides the best data access mechanism, by type of query, while applying a. Basically, the rbo used a set of rules to determine how to execute a query. Query optimization in dbms query optimization in sql. Oracle query optimization tool the query optimizer, is a builtin database software which helps in the optimization of sql queries in oracle, by finding out the most efficient method in which a sql query can access data from an oracle database. The execution plan selected for a sql statement is just one of the many alternative execution plans considered. In order to solve this problem, we need to provide. It discusses a suite of heuristic and costbased transformations performed by oracle.
They go by different names in different engines, so ill use the microsoft names since thats what i am most familiar with. Annotate resultant expressions to get alternative query plans. By applying this model to the statistics about your data, the optimizer tries to efficiently convert your query into an executable plan. For any sql statement processed by oracle database, the optimizer. This company is probably taken over by quest software and i faced a lot of bugs in the trial version of quest sql optimizer 8. You should also use the cbo for data warehousing applications, because the cbo supports enhanced features for dss. Oracle corporation is continually improving the cbo and new features require cbo. The operation can be scanning a table, accessing rows from a table by using an index, joining two tables together, or sorting a row set. Plsql will be in our sights in the next part but suffice to say it is the key to maintainable code on oracle. The query optimizer chooses the plan with the lowest estimated cost. While the exact workings of the oracle cost based optimizer are a proprietary secret of oracle corporation, the basics of query optimization in general, and cost calculation, selectivity determination, and cardinality estimation in particular, have been extensively studied in research labs and universities since e. The oracle server provides the costbased cbo and rulebased rbo optimization. Annotate resultant expressions to get alternative query plans 3.
A single query can be executed through different algorithms or rewritten in different forms and structures. Generate logically equivalent expressions using equivalence rules 2. Query optimization is less efficient when date statistics are not correctly updated. The query can use different paths based on indexes, constraints, sorting methods etc. Although oracle supports the rule based optimizer, you should design new applications to use the cost based optimizer cbo. To get you started, there is a pdf of chapter 5 clustering factor that you can download from apress. Introduced in 1992 with oracle7, the costbased optimizer has been continually enhanced and. Costbased query optimization assign cost to operations assign cost to partial or alternative plans search for plan with lowest cost costbased optimizations. Oracle performance tuning interview questions and answers. Cost based optimizer also checks for the size of the table and the number of records returned by the query, so if the table is small or if the query returns more than 15% to 20% of the total number of records then oracle prefers full table scan over index scan. The query optimizer, which carries out this function, is a key part of the relational database and determines the most efficient way to access data. Query optimization techniques in microsoft sql server. There are several stages in executing a query that you submit to any sql dbms. It has b een studied in a great v ariet y of con texts and from man y di eren t angles, giving rise to sev eral div erse solutions in eac h case.
By far the biggest change to the optimizer in oracle database 12c is adaptive query optimization. General idea access method join order subquery strategy t2 t3 t1 table scan range scan ref access join join. The seminal paper on costbased query optimization is 15. Typically cost based is better, but does have the drawback of requiring that statistics be kept fairly up to date, but this drawback has become less of an issue as the underlying hardware has gotten better. If the flag is on the default, the optimizer performs a costbased choice between subquery materialization and intoexists subquery transformation if either method could be used.
This may involved rewriting rulebased sql, especially queries where you rely on the ordering of the table names in the from clause to determine the driving. A long time ago, the only optimizer in the oracle database was the rulebased optimizer rbo. Its important to have statistics on all tables for the cbo cost based optimizer to work correctly. Costbased query optimizers evaluate the resource footprint of various query. Cost based optimizer cbo is based on the cost associated with the query whereas rule based optimizer rbo is based on certain specific rules. The focus, however, is on query optimization in centralized database systems. Overview catalog information for cost estimation measures of query cost selection join operations other operations evaluation and transformation. A query plan or query execution plan is an ordered set of steps used to access data in a sql relational database management system.
Rulebased sql optimizer rbo desupported in oracle10g. A query is a request for information from a database. Note that without properly collected statistics, the cbo will do one of two things. Query optimization in relational algebra geeksforgeeks. Query optimization is a feature of many relational database management systems. Query optimization an overview sciencedirect topics. A cost estimation technique so that a cost may be assigned to each plan in the search space. The optimizer selects the execution plan with the lowest cost, where cost represents the estimated resource usage for that plan. You can find the cost of the query using the using explain plan for. Query optimization in oracle database 10g release 2. Oracle uses a costbased approach for several classes of query transformations.
In oracle database 7, the cost based optimizer cbo was introduced to deal with the enhanced functionality being added to the oracle database at this time, including parallel execution and partitioning, and to take the actual data content and distribution into account. Cost based optimization physical this is based on the cost of the query. A solid understanding of the optimizer is essential for sql tuning. Cost based optimizer considers the statastical information of the table for query execution. So, the cost used by the query optimizer represents an estimate of the number of disk ios and the amount of cpu and memory used in performing an operation.
Thus, query optimization can be viewed as a difficult search problem. The cbo has evolved into one of the worlds most sophisticated software components, and it has the challenging job of evaluating any sql statement and generating the best execution plan for the statement. It can be done in many ways like you can set cbo at the statement level using hints. The optimizer determines the most efficient execution plan for each sql statement based on the structure of the given query, the available.
Query optimization in oracle9i oracle integrated cloud. Cost difference between evaluation plans for a query can be enormous e. The following sections discuss several examples of oracles transformation technologies. Pdf this paper describes costbased query transformation in oracle relational database system, which is a novel phase in query optimization. The output from the optimizer is a plan that describes an optimum method of execution.
The oracle costbased optimizer cbo displays the cost number for a query, or an estimate based on statistics and calculations. In the plsql challenge optimization series, weve been using autotrace. With nearly 50% of production systems using rulebased sql optimization rbo, this is big for oracle. The cost model will chose the scenario for least cost and most efficient way to run the query. Understand the basic concepts underlying the steps in query processing and optimization and estimating query processing cost. Obviously, adhoc queries cannot benefit from packages, but at least they profit from having solid access structures, which are of course important to plsql too.
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