# Tracking column histogram modifications by M.Houri
# https://hourim.wordpress.com/2020/08/06/historical-column-histogram/


create table T1 tablespace TS1 as  
select rownum id, decode(mod(rownum,10),0,2,1) c_freq, nvl(blocks,999) c_hb  
from dba_tables ; 

update T1 set c_freq=3 where rownum<=10; 
commit; 

create index idx_freq on T1(C_FREQ) tablespace TS1; 
create index idx_hb on T1(C_HB) tablespace TS1; 


select c_freq,count(*) from T1 group by c_freq order by 2 desc; 


exec dbms_stats.gather_table_stats (user, 'T1', method_opt=>'for all columns size 1'); 

col column_name for a20

select column_name,num_distinct,density,num_nulls,num_buckets,sample_size,histogram  
from user_tab_col_statistics  
where table_name='T1' and column_name='C_FREQ'; 



select  /*+ GATHER_PLAN_STATISTICS */ * from T1 where C_FREQ=3; 

select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST  +PEEKED_BINDS +PARALLEL +PARTITION +COST +BYTES'));


select column_name,num_distinct,density,num_nulls,num_buckets,sample_size,histogram  
from user_tab_col_statistics  
where table_name='T1' and column_name='C_HB'; 

select /*+ GATHER_PLAN_STATISTICS */ * from T1 where C_HB=999; 
select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST  +PEEKED_BINDS +PARALLEL +PARTITION +COST +BYTES'));


---------------- FREQ

exec dbms_stats.gather_table_stats(user,'T1', method_opt=>'for columns C_FREQ size AUTO');

select column_name,num_distinct,density,num_nulls,num_buckets,sample_size,histogram  
from user_tab_col_statistics  
where table_name='T1' and column_name='C_FREQ'; 

select endpoint_value as column_value, 
endpoint_number as cummulative_frequency, 
endpoint_number - lag(endpoint_number,1,0) over (order by endpoint_number) as frequency 
from user_tab_histograms 
where table_name = 'T1' and column_name = 'C_FREQ'; 

alter system flush shared_pool;

select  /*+ GATHER_PLAN_STATISTICS */ * from T1 where C_FREQ=3; 

select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST  +PEEKED_BINDS +PARALLEL +PARTITION +COST +BYTES'));


--------------- WEIGHT

exec dbms_stats.gather_table_stats(user,'T1', method_opt=>'for columns C_HB size 254');

select column_name,num_distinct,density,num_nulls,num_buckets,sample_size,histogram  
from user_tab_col_statistics  
where table_name='T1' and column_name='C_HB'; 


select endpoint_value as column_value, 
endpoint_number as cummulative_frequency, 
endpoint_number - lag(endpoint_number,1,0) over (order by endpoint_number) as frequency 
from user_tab_histograms 
where table_name = 'T1' and column_name = 'C_HB'; 



create table T1 tablespace TS1 as  
select rownum id, decode(mod(rownum,10),0,2,1) c_freq, nvl(blocks,999) c_hb  
from dba_extents ; 

update T1 set c_freq=3 where rownum<=10; 
commit;