MySQL :: MySQL 8.4 参考手册 :: 10.15.10 选择要跟踪的优化器功能

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MySQL 8.4 参考手册 / 10 优化 / 10.15 跟踪优化器 / 10.15.10 选择要跟踪的优化器功能

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10.15.10 选择要跟踪的优化器功能10.15.10 Selecting Optimizer Features to Trace

优化器中的某些功能在语句优化和执行过程中可能会被多次调用，因此可能导致跟踪信息膨胀得不合理。它们包括：Some features in the optimizer can be invoked many times during statement optimization and execution, and thus can make the trace grow beyond reason. They are:

贪婪搜索：对于一个N表连接，这可能会探索 factorial(N) 个计划。Greedy search: With an N-table join, this could explore factorial(N) plans.
范围优化器Range optimizer
动态范围优化：在 EXPLAIN 输出中显示为 range checked for each record；每个外部行都会导致范围优化器重新运行。Dynamic range optimization: Shown as range checked for each record in EXPLAIN output; each outer row causes a re-run of the range optimizer.
子查询：在子查询中，WHERE 子句可能会对每一行执行一次。Subqueries: A subquery in which the WHERE clause may be executed once per row.

可以通过将optimizer_trace_features系统变量的一个或多个开关设置为OFF来排除这些功能的跟踪。以下是这些开关的列表：Those features can be excluded from tracing by setting one or more switches of the optimizer_trace_features system variable to OFF. These switches are listed here:

greedy_search：贪心搜索不被跟踪。greedy_search: Greedy search is not traced.
range_optimizer：范围优化器不被跟踪。range_optimizer: The range optimizer is not traced.
dynamic_range：仅跟踪此JOIN_TAB::SQL_SELECT上范围优化器的第一次调用。dynamic_range: Only the first call to the range optimizer on this JOIN_TAB::SQL_SELECT is traced.
repeated_subselect：仅跟踪此Item_subselect的第一次执行。repeated_subselect: Only the first execution of this Item_subselect is traced.

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