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Oracle9i, 10g 优化模式 OPTIMIZER_MODE

原创 Linux操作系统 作者:tolywang 时间:2009-07-15 17:00:37 0 删除 编辑



10g  Values:        default  all_rows

first_rows_n
The optimizer uses a cost-based approach and optimizes with a goal of best response time to return the first n rows (where n = 1, 10, 100, 1000).

first_rows
The optimizer uses a mix of costs and heuristics to find a best plan for fast delivery of the first few rows.

all_rows
The optimizer uses a cost-based approach for all SQL statements in the session and optimizes with a goal of best throughput (minimum resource use to complete the entire statement).


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9i Values:          default  choose 

rule
The optimizer chooses a rule-based approach for all SQL statements regardless of the presence of statistics.

choose
The optimizer chooses between a cost-based approach and a rule-based approach based on whether statistics are available.

If the data dictionary contains statistics for at least one of the accessed tables, then the optimizer uses a cost-based approach and

optimizes with a goal of best throughput. If the data dictionary contains only some statistics, then the cost-based approach is used, and the optimizer must guess the statistics for the subjects without any statistics. This can result in sub-optimal execution plans. If the data dictionary contains no statistics for any of the accessed tables, then the optimizer uses a rule-based approach.


first_rows_n
The optimizer uses a cost-based approach, regardless of the presence of statistics, and optimizes with a goal of best response time to return the first n rows (where n = 1, 10, 100, 1000).


first_rows
The optimizer uses a mix of costs and heuristics to find a best plan for fast delivery of the first few rows.


all_rows
The optimizer uses a cost-based approach for all SQL statements in the session regardless of the presence of statistics and optimizes with a goal of best throughput (minimum resource use to complete the entire statement).

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