Why access rows to realize that only rows are needed? Sometimes indexes can be extremely inefficient. In this case, you should examine the following: All statistics, except the number of output rows, are cumulative.
This 10 minutes step by step primer is the first of a two part article that will teach you exactly the things you must know about Query Plan. What is Query Execution Plan? When you fire an SQL query to Oracle, Oracle database internally creates a query execution plan in order to fetch the desired data from the physical tables.
The query execution plan is nothing but a set of methods on how the database will access the data from the tables. This query execution plan is crucial as different execution plans will need different cost and time for the query execution. How the Execution Plan is created actually depends on what type of query optimizer is being used in your Oracle database.
For Oracle 10g, CBO is the default optimizer.
Cost Based optimizer enforces Oracle to generate the optimization plan by taking all the related table statistics into consideration. On the other hand, RBO uses a fixed set of pre-defined rules to generate the query plan.
Obviously such fixed set of rules may not always be able to create the plan that is most efficient in nature. This article will briefly help you understand, How can we see Query Execution Plan How do we understand or rather interpret the execution plan. Remember, currently there is no index on the table.
This means, in order to fetch the id value as requested, Oracle is actually reading the index only and not the whole table. Oracle can then use this rowid to fetch further information if requested in query.
And then it selects all the columns by the rowid. The same thing will normally happen if we use a between clause also.Jan 16, · Remember, too, that EXPLAIN PLAN is an estimated execution path for the query, based upon cursory examination of relevant statistics; the query is not executed so it doesn't go through the same process of parsing and plan generation as an executed query would.
If you have been in my tuning class or read my tuning blogs, I carry on at length about the importance of reviewing the explain plan the SQL executed with, not an explain plan because you asked for one.
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Generate Explain Plan s. The explain plan displays the steps a database uses to execute an SQL statement. The results of the plan display the order a database uses to search/join tables, the types of access used (indexed search or full table scan), and the names of indexes used.
This means, if a query contains a bind variable and if the execution plan of that query is dependant on the value provided to the bind, then the execution plan displayed (or generated) by “explain plan” / “autotrace” is not guaranted to be the same as that of runtime optimizer.
EXPLAIN PLAN parses a query and records the "plan" that Oracle devises to execute it. By examining this plan, you can find out if Oracle is picking the right indexes and joining your tables in the most efficient manner.
There are a few different ways to utilize Explain Plan.