Optimizer statistics include table, column, index, and system statistics.
Statistics for tables and indexes are stored in the data dictionary. These
statistics are not intended to provide real-time data. They provide the
optimizer a statistically correct snapshot of data storage and
distribution, which the optimizer uses to make decisions on how to access data.
(indexes only)
The statistics that are collected include:
·
Size of the table or index in database blocks
·
Number of rows
·
Average row size and chain count (tables only)
·
Height and number of deleted leaf rows
(indexes only)
As data is inserted, deleted, and modified, these facts change. Because
the performance impact of maintaining real-time data distribution statistics is
prohibitive, these statistics are updated by periodically gathering statistics
on tables and indexes.
Optimizer statistics are collected automatically by an automatic
maintenance job that runs during predefined maintenance windows once daily by
default. System statistics are operating system characteristics that are used
by the optimizer. These statistics are not collected automatically. For details
about collecting system statistics
Optimizer statistics are not the same as the database performance
statistics that are gathered in the AWR snapshot.
