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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