查看数据分布
select owner,count(*) from test group by owner order by 2 desc;
查看列的基数和选择性
统计信息不准确
（运行下面的SQL首先应该能检查该表的 segment_size 有多大，如果segment_size超过超过SGA的buffer_cache，会影响系统）
select count(distinct column_name),
count(*) total_rows,
count(distinct column_name) / count(*) * 100 selectivity
from table_name;
统计信息准确
select a.column_name,
a.num_distinct cardinality,
round(a.num_distinct / b.num_rows * 100, 2) selectivity
from dba_tab_col_statistics a, dba_tables b
where a.owner = b.owner
and a.table_name = b.table_name
and a.owner = upper('&owner')
and a.table_name = upper('&table_name')
and a.column_name = upper('&column_name');