優化必懂概念-基數

1、概念 某一列唯一鍵的數量叫做基數。 2、舉例 性別列，該列只分男女，so 這一列的基數爲2。 3、測試 SYS@orcl> select count(distinct owner),count(distinct object_id),count(*) from t; COUNT(DISTINCTOWNER) COUNT(DISTINCTOBJECT_ID) COUNT(*) -------------------- ------------------------ ---------- 20 83962 83962 t表的總行數爲83962，owner列的基數爲20，object_id的基數83962，說明owner列有大量的重複值。object_id很有可能是主鍵。 SYS@orcl> select owner,count(*) from t group by owner order by 2 desc; OWNER COUNT(*) ------------------------------ ---------- SYS 37365 PUBLIC 33339 SYSMAN 3554 APEX_030200 2561 ORDSYS 2513 MDSYS 1447 XDB 1167 SYSTEM 611 CTXSYS 389 WMSYS 333 EXFSYS 312 ORDDATA 257 DBSNMP 57 FLOWS_FILES 13 OUTLN 10 ORDPLUGINS 10 ORACLE_OCM 8 SI_INFORMTN_SCHEMA 8 APPQOSSYS 5 ROIDBA 3 20 rows selected. SYS@orcl>