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approx_count_distinct tips

Oracle Tips by Donald BurlesonMay 25, 2017

Question: What does 12c approx_count_distinct do, and how do I use approx_count_distinct to get better schema statistics?  Are there other uses for this new function?

Answer:  The approx_count_distinct is a 12c new feature to reduces the time required to get the number of distinct values (NDV) for a table column.  This approx_count_distinct was added in 11g (but not documented), as an aid in speeding-up dbms_stats collection of column distinct values.

exec dbms_stats.gather_table_stats(‘my_schema',’my_tab’);

Tests indicate the approx_count_distinct is more than eight times faster than the traditional count distinct, and the speed performances are even greater for massive data warehouse tables.

Statistically, the approx_count_distinct approximations provide a statistically insignificant difference from count distinct, so the approximation is statistically valid.

In a traditional count distinct, Oracle's read consistency mechanism is invoked, causing a large time lag when counting the number of distinct values in a very large table.  Also, as the number of distinct values increase, the elapsed time and memory usage of the count distinct increases drastically.

In contrast, the approx_count_distinct bypasses the read consistency mechanism and give a fast and relatively accurate approximation of the number of distinct values in a table column.

The approx_count_distinct can also be used to quickly count the number of tables.

alter system set approx_for_count_distinct = TRUE;
select

   tablespace_name,
  APPROX_COUNT_DISTINCT(table_name) as tab_count
from
   dba_tables
group by
   tablespace_name
order by
   tablespace_name;



   
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