Why is GATHER STATS AUTO_SAMPLE_SIZE always 100% (Compute) in 12c

Hello at all,

we have a strange phenomenon in our databases.

The statistic collection of our tables starts usually in the automatic maintenance windows.

We use the default with auto_sample_size:

ESTIMATE_PERCENT    DBMS_STATS.AUTO_SAMPLE_SIZE

SInce we graded up our databases to 12c from 11gr2 we realized that we need much more time for gathering table stats for our tables.

In the past we never need more than about 2h for our biggest tables.

Since 12c we need for the same tables about 8h.

We saw the auto sample size in dba_tables was the same like the num_rows. So we guess the sample auto uses 100% (compute statistics) to analyze our tables.