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Oracle: Init parameter STATISTICS_LEVEL

The value of statistics_level can be set to
all
typical The default value. It collects all statistics except timed operating system statistics and plan execution statistics (aka row source statistics).
basic Disables the collection of some statistics.
The list of statistics that can be enabled or disabled with statistics_level can be queried from v$statistics_level.

Relationship to TIMED_STATISTICS

The value of timed_statistics must be true if statistics_level is changed to typical or all.
Vice versa, timed_statistics cannot be set to false if statistics_level is typical or all. 
select p.name, p.value
from v$parameter p
where p.name in ( 'statistics_level', 'timed_statistics' );
--
-- NAME             VALUE
-- ---------------- -------
-- timed_statistics TRUE
-- statistics_level TYPICAL

alter session set timed_statistics = false;
> ORA-02097: parameter cannot be modified because specified value is invalid
> ORA-00044: timed_statistics must be TRUE when statistics_level is not BASIC

Relationship to _ROWSOURCE_STATISTICS_SAMPFREQ

Jonathan Lewis suspects that setting statistics_level to all implicitly sets _rowsource_statistics_sampfreq to 1.

Misc

In order to enable automatich shared memory management, the value of statistics_level must either be typical or all.

See also

Prerequisites for filling V$SQL_PLAN_STATISTICS and V$SQL_PLAN_STATISTICS_ALL
timed_statistics, timed_os_statistics
ORA-00044: timed_statistics must be TRUE when statistics_level is not BASIC

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