statistics:sys,table,index,column
package:dbms_stats (exec dbms_stats.gather_table_stats('HR','T'))
dba_tables dba_indexes
v$user_tab_statistics v$user_tab_col_statistics
v$segstat_name v$segstat v$segment_statistics
para: optimizer_dynamic_sampling
dbms_stats.gather_table_stats -> dba_histograms dba_tab_histograms
histograms:width-balanced,height-balanced
monitor table
optimizer cost model:cpu_cost,io_cost,temp_space
export import statistics between database