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Execution plans can differ due to the following: For example, in the following explain plan, the last step is a very unselective range scan that is executed 76563 times, accesses 11432983 rows, throws away 99% of them, and retains 76563 rows.

Why access 11432983 rows to realize that only 76563 rows are needed?

Rows Execution Plan -------- ---------------------------------------------------- 12 SORT AGGREGATE 2 SORT GROUP BY 76563 NESTED LOOPS 76575 NESTED LOOPS 19 TABLE ACCESS FULL CN_PAYRUNS_ALL 76570 TABLE ACCESS BY INDEX ROWID CN_POSTING_DETAILS_ALL 76570 INDEX RANGE SCAN (object id 178321) 76563 TABLE ACCESS BY INDEX ROWID CN_PAYMENT_WORKSHEETS_ALL 11432983 INDEX RANGE SCAN (object id 186024) output that shows that a statement uses an index does not necessarily mean that the statement runs efficiently. In this case, you should examine the following: to determine an access plan, and then later prove that it is the optimal plan through testing.

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A statement's execution plan is the sequence of operations Oracle performs to run the statement.The row source tree is the core of the execution plan.It shows the following information: results let you determine whether the optimizer selects a particular execution plan, such as, nested loops join.It also helps you to understand the optimizer decisions, such as why the optimizer chose a nested loops join instead of a hash join, and lets you understand the performance of a query.output shows how Oracle runs the SQL statement when the statement was explained.This can differ from the plan during actual execution for a SQL statement, because of differences in the execution environment and explain plan environment.