Find INTCOL#=1001 in col_usage$?
在 <了解你所不知道的SMON功能(四):维护col_usage$字典基表>中我介绍了SMON后台进程维护字典基表COL_USAGE$一些细节，有网友阅读了这篇文档后发现其数据库的COL_USAGE$中存在INTCOL#=1001的记录。 INTCOL#列表示internal column number对应于COL$基表的INTCOL#，注意Internal Column Number与COL#(column number as created)是不同的。$ORACLE_HOME/rdbms/admin/sql.bsq对于INTCOL#给出了解释:
* If a table T(c1, addr, c2) contains an ADT column addr which is stored * exploded, the table will be internally stored as * T(c1, addr, C0003$, C0004$, C0005$, c2) * Of these, only c1, addr and c2 are user visible columns. Thus, the * user visible column numbers for (c1, addr, C0003$, C0004$, C0005$, c2) * will be 1,2,0,0,0,3. And the corresponding internal column numbers will * be 1,2,3,4,5,6. * * Some dictionary tables like icol$, ccol$ need to contain intcol# so * that we can have indexes and constraints on ADT attributes. Also, these * tables also need to contain col# to maintain backward compatibility. * Most of these tables will need to be accessed by col#, intcol# so * indexes are created on them based on (obj#, col#) and (obj#, intcol#). * Indexes based on col# have to be non-unique if ADT attributes might * appear in the table. Indexes based on intcol# can be unique.这里的ADT指的是抽象数据类型(Abstract DataType is a user defined data type)，例如:
CREATE OR REPLACE TYPE persons_address AS OBJECT ( streetNumber NUMBER, streetName VARCHAR2(30), citySuburb VARCHAR2(30), state VARCHAR2(4), postCode NUMBER );熟读Oracle官方文档的朋友一定会记得，Oracle中单表的column总数存在一个上限:1000，即单表不能拥有超过1000个列。 但令人疑惑的是INTCOL#居然是1001，显然1001是某种magic number，而不是指第1001列。 搞清楚这个问题后，再进一步探索就不难发现问题的关键了:
SQL> select * from v$version;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi
PL/SQL Release 10.2.0.4.0 - Production
CORE 10.2.0.4.0 Production
TNS for Linux: Version 10.2.0.4.0 - Production
NLSRTL Version 10.2.0.4.0 - Production
SQL> select * from global_name;
GLOBAL_NAME
--------------------------------------------------------------------------------
www.oracledatabase12g.com
SQL> drop table maclean;
Table dropped.
SQL> create table maclean(oppo_find_me int);
Table created.
SQL> select object_id from dba_objects where object_name='MACLEAN';
OBJECT_ID
----------
1343832
SQL> select intcol# from col_usage$ where obj#=1343832;
no rows selected
SQL> insert into maclean values(1);
1 row created.
SQL> commit;
Commit complete.
SQL> select rowid from maclean;
ROWID
------------------
AAFIFYAABAAByPKAAA
SQL> delete maclean where rowid='AAFIFYAABAAByPKAAA';
1 row deleted.
SQL> commit;
Commit complete.
SQL> exec DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO;
PL/SQL procedure successfully completed.
SQL> select intcol#,equality_preds from col_usage$ where obj#=1343832;
INTCOL# EQUALITY_PREDS
---------- --------------
1001 1
通过这一点可以了解那些字典基表是以rowid为条件来查询或更新的
SQL> select owner || '.' || object_name
2 from dba_objects
3 where object_id in (select obj# from col_usage$ where intcol# = 1001);
OWNER||'.'||OBJECT_NAME
--------------------------------------------------------------------------------
SYS.COL$
SYS.CDEF$
SYS.VIEW$
SYS.SUMPARTLOG$
SYS.SUM$
SYS.SUMKEY$
SYS.SUMAGG$
SYS.SUMPRED$
SYS.SUMQB$
SYS.PS$
SYS.AW_OBJ$
OWNER||'.'||OBJECT_NAME
--------------------------------------------------------------------------------
SYS.AW_PROP$
SYS.WRI$_ADV_OBJECTS
WMSYS.WM$WORKSPACES_TABLE
SYS.MACLEAN
CTXSYS.DR$INDEX
XDB.XDB$H_INDEX
XDB.XDB$RESOURCE
EXFSYS.RLM$SCHACTLIST
SYS.AW$EXPRESS
MACLEAN.SAMPLE 总结: Oracle最早在9i中引入了col_usage$字典基表，其目的在于监控column在SQL语句作为predicate的情况，col_usage$的出现完善了CBO中柱状图自动收集的机制。该字典基表上的INTCOL#列代表Internal Column Number以标识ADT列。
INTCOL#等于1001代表ROWID伪列，也就是相关对象曾使用ROWID充当predicate。