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Oracle Interview Questions And Answers
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Page 11 of 95
101.
Will
the
Optimizer
always
use
COST-based
approach
if
OPTIMIZER_MODE is set to "Cost'? Presence of statistics in the data dictionary for atleast one of the tables accessed by the SQL statements is necessary for the OPTIMIZER to use COST-based approach. Otherwise OPTIMIZER chooses RULE- based approach. 102. What is the effect of setting the value of OPTIMIZER_MODE to 'RULE' ? This value causes the optimizer to choose the rule_based approach for all SQL statements issued to the instance regardless of the presence of statistics. 103. What are the values that can be specified for OPTIMIZER_GOAL parameter of the ALTER SESSION Command ? CHOOSE,ALL_ROWS,FIRST_ROWS and RULE. 104. What is the effect of setting the value "CHOOSE" for OPTIMIZER_GOAL, parameter of the ALTER SESSION Command ? The Optimizer chooses Cost_based approach and optimizes with the goal of best throughput if statistics for atleast one of the tables accessed by the SQL statement exist in the data dictionary. Otherwise the OPTIMIZER chooses RULE_based approach. 105. What is the effect of setting the value "ALL_ROWS" for OPTIMIZER_GOAL parameter of the ALTER SESSION command ? This value causes the optimizer to the cost-based approach for all SQL statements in the session regardless of the presence of statistics and to optimize with a goal of best throughput. 106. What is the effect of setting the value 'FIRST_ROWS' for OPTIMIZER_GOAL parameter of the ALTER SESSION command ? This value causes the optimizer to use the cost-based approach for all SQL statements in the session regardless of the presence of statistics and to optimize with a goal of best response time. 107. What is the effect of setting the 'RULE' for OPTIMIER_GOAL parameter of the ALTER SESSION Command ? This value causes the optimizer to choose the rule-based approach for all SQL statements in a session regardless of the presence of statistics. 108. What is RULE-based approach to optimization ? Choosing an executing planbased on the access paths available and the ranks of these access paths. 109. What is COST-based approach to optimization ? Considering available access paths and determining the most efficient execution plan based on statistics in the data dictionary for the tables accessed by the statement and their associated clusters and indexes. |