优化必懂概念-基数

1、概念某一列唯一键的数量叫做基数。2、举例性别列，该列只分男女，so 这一列的基数为2。3、测试SYS@orcl> select count(distinct owner),count(distinct object_id),count(*) from t;COUNT(DISTINCTOWNER) COUNT(DISTINCTOBJECT_ID)   COUNT(*)-------------------- ------------------------ ----------                  20                    83962      83962    t表的总行数为83962，owner列的基数为20，object_id的基数83962，说明owner列有大量的重复值。object_id很有可能是主键。    SYS@orcl> select owner,count(*) from t group by owner order by 2 desc;OWNER                            COUNT(*)------------------------------ ----------SYS                                 37365PUBLIC                              33339SYSMAN                               3554APEX_030200                          2561ORDSYS                               2513MDSYS                                1447XDB                                  1167SYSTEM                                611CTXSYS                                389WMSYS                                 333EXFSYS                                312ORDDATA                               257DBSNMP                                 57FLOWS_FILES                            13OUTLN                                  10ORDPLUGINS                             10ORACLE_OCM                              8SI_INFORMTN_SCHEMA                      8APPQOSSYS                               5ROIDBA                                  320 rows selected.SYS@orcl>