10G默认optimizer_mode为all_rows

最新推荐文章于 2022-11-13 02:23:16 发布

cneo2012

最新推荐文章于 2022-11-13 02:23:16 发布

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本文详细介绍了Oracle数据库中优化器模式的各种设置及其作用。包括first_rows_n、first_rows和all_rows等不同模式的选择依据和优化目标，这对于理解SQL执行计划及优化数据库性能至关重要。

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show">sys@ora10g>show parameter optimizer_mode

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
optimizer_mode string all_rows
optimizer_mode establishes the default behavior for choosing an optimization
approach for the instance.
Values:
■ 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).