Must understand the concept of optimization-cardinality

1. The number of unique keys in a column is called cardinality. 2. For example, the gender column is divided into men and women only, and the cardinality of the so column is 2. 3. Test SYS@orcl > select count (distinct owner), count (distinct object_id), count (*) from t COUNT (DISTINCTOWNER) COUNT (DISTINCTOBJECT_ID) COUNT (*)-the total number of rows in the 20 83962 83962 t table is 83962. The cardinality of the owner column is 83962 of the 20 owner column, indicating that there are a large number of duplicate values in the owner column. Object_id is most likely the primary key. 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 >