查看数据分布 & 查看列的基数和选择性

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');