Ask about Histogram after gather statistics.

Hello

My oracle db version is 11.2.0.4 in AIX

I have used DBMS_STATS.GATHER_SCHEMA_STATS to gather statistics all tables in a schema with below options:

estimate_percent => DBMS_STATS.AUTO_SAMPLE_SIZE,

granularity => 'AUTO',

method_opt => 'FOR ALL COLUMNS SIZE AUTO',

no_invalidate => FALSE,

cascade => TRUE

But when I query information from DBA_TAB_COLUMNS, there are 2 things I don't understand why:

1. Oracle did not analyze and update statistics for COL_C and COL_D (last_analyzed is 11 Oct, others are 15 Oct). ?
2. And event the 2 first columns were analyzed but value of Histogram is NONE (total records of this table is 245878223). I guest these columns having large of number NULL values so Histogram = NONE?