Know Difference between Oracle Reserved Words and Keywords

与C和JAVA体系类似,Oracle中存在一些保留词(Reserved Words)或关键词(Keywords)以及少量的命名空间(Namespace)。

保留词(Reserved Words)是优先级最高的词汇,在Oracle中这些词都有着特殊的意义。所以这些词不允许被重定义 (redefine),换而言之就不是不允许用作对象的名字。以下为Oracle 10g中保留词的列表:

Oracle Reserved Words
ACCESS ELSE MODIFY START
ADD EXCLUSIVE NOAUDIT SELECT
ALL EXISTS NOCOMPRESS SESSION
ALTER FILE NOT SET
AND FLOAT NOTFOUND SHARE
ANY FOR NOWAIT SIZE
ARRAYLEN FROM NULL SMALLINT
AS GRANT NUMBER SQLBUF
ASC GROUP OF SUCCESSFUL
AUDIT HAVING OFFLINE SYNONYM
BETWEEN IDENTIFIED ON SYSDATE
BY IMMEDIATE ONLINE TABLE
CHAR IN OPTION THEN
CHECK INCREMENT OR TO
CLUSTER INDEX ORDER TRIGGER
COLUMN INITIAL PCTFREE UID
COMMENT INSERT PRIOR UNION
COMPRESS INTEGER PRIVILEGES UNIQUE
CONNECT INTERSECT PUBLIC UPDATE
CREATE INTO RAW USER
CURRENT IS RENAME VALIDATE
DATE LEVEL RESOURCE VALUES
DECIMAL LIKE REVOKE VARCHAR
DEFAULT LOCK ROW VARCHAR2
DELETE LONG ROWID VIEW
DESC MAXEXTENTS ROWLABEL WHENEVER
DISTINCT MINUS ROWNUM WHERE
DROP MODE ROWS WITH

注意虽然我们不能直接将保留词用于对象名,但还是可以通过特殊手段来完成这种不推荐的定义目的,如:

如上表所列START为一个保留词
SQL> CREATE TABLE START (T1 INT);

CREATE TABLE START (T1 INT)

ORA-00903: invalid table name

利用双引号可以成功将保留词用作对象名,但是在管理时会很麻烦,极力不推荐!

SQL> CREATE TABLE "START" (T1 INT);

Table created

SQL> SELECT * FROM START;

SELECT * FROM START

ORA-00903: invalid table name

SQL> SELECT * FROM "START";

                                     T1
---------------------------------------

SQL> SELECT * FROM "start";

SELECT * FROM "start"

ORA-00942: table or view does not exist

SQL> select object_name from dba_objects where object_name='START';

OBJECT_NAME
--------------------------------------------------------------------------------
START

SQL> DROP TABLE "start";

DROP TABLE "start"

ORA-00942: table or view does not exist

SQL> DROP TABLE "START";

Table dropped

 

关键词同样是在Oracle中具有特殊意义的词汇,但是其可以用作重定义(redefine)。Oracle文档介绍这些关键词可能在今后的版本中变成保留词。以下为Oracle 10g中关键词的列表:

Oracle Keywords
ADMIN CURSOR FOUND MOUNT
AFTER CYCLE FUNCTION NEXT
ALLOCATE DATABASE GO NEW
ANALYZE DATAFILE GOTO NOARCHIVELOG
ARCHIVE DBA GROUPS NOCACHE
ARCHIVELOG DEC INCLUDING NOCYCLE
AUTHORIZATION DECLARE INDICATOR NOMAXVALUE
AVG DISABLE INITRANS NOMINVALUE
BACKUP DISMOUNT INSTANCE NONE
BEGIN DOUBLE INT NOORDER
BECOME DUMP KEY NORESETLOGS
BEFORE EACH LANGUAGE NORMAL
BLOCK ENABLE LAYER NOSORT
BODY END LINK NUMERIC
CACHE ESCAPE LISTS OFF
CANCEL EVENTS LOGFILE OLD
CASCADE EXCEPT MANAGE ONLY
CHANGE EXCEPTIONS MANUAL OPEN
CHARACTER EXEC MAX OPTIMAL
CHECKPOINT EXPLAIN MAXDATAFILES OWN
CLOSE EXECUTE MAXINSTANCES PACKAGE
COBOL EXTENT MAXLOGFILES PARALLEL
COMMIT EXTERNALLY MAXLOGHISTORY PCTINCREASE
COMPILE FETCH MAXLOGMEMBERS PCTUSED
CONSTRAINT FLUSH MAXTRANS PLAN
CONSTRAINTS FREELIST MAXVALUE PLI
CONTENTS FREELISTS MIN PRECISION
CONTINUE FORCE MINEXTENTS PRIMARY
CONTROLFILE FOREIGN MINVALUE PRIVATE
COUNT FORTRAN MODULE PROCEDURE
PROFILE SAVEPOINT SQLSTATE TRACING
QUOTA SCHEMA STATEMENT_ID TRANSACTION
READ SCN STATISTICS TRIGGERS
REAL SECTION STOP TRUNCATE
RECOVER SEGMENT STORAGE UNDER
REFERENCES SEQUENCE SUM UNLIMITED
REFERENCING SHARED SWITCH UNTIL
RESETLOGS SNAPSHOT SYSTEM USE
RESTRICTED SOME TABLES USING
REUSE SORT TABLESPACE WHEN
ROLE SQL TEMPORARY WRITE
ROLES SQLCODE THREAD WORK
ROLLBACK SQLERROR TIME

 

Pl/SQL中还有部分的保留词(PL/SQL Reserved Words),在嵌入SQL语句需要对这些词汇特殊处理。

PL/SQL Reserved Words
ABORT BETWEEN CRASH DIGITS
ACCEPT BINARY_INTEGER CREATE DISPOSE
ACCESS BODY CURRENT DISTINCT
ADD BOOLEAN CURRVAL DO
ALL BY CURSOR DROP
ALTER CASE DATABASE ELSE
AND CHAR DATA_BASE ELSIF
ANY CHAR_BASE DATE END
ARRAY CHECK DBA ENTRY
ARRAYLEN CLOSE DEBUGOFF EXCEPTION
AS CLUSTER DEBUGON EXCEPTION_INIT
ASC CLUSTERS DECLARE EXISTS
ASSERT COLAUTH DECIMAL EXIT
ASSIGN COLUMNS DEFAULT FALSE
AT COMMIT DEFINITION FETCH
AUTHORIZATION COMPRESS DELAY FLOAT
AVG CONNECT DELETE FOR
BASE_TABLE CONSTANT DELTA FORM
BEGIN COUNT DESC FROM
FUNCTION NEW RELEASE SUM
GENERIC NEXTVAL REMR TABAUTH
GOTO NOCOMPRESS RENAME TABLE
GRANT NOT RESOURCE TABLES
GROUP NULL RETURN TASK
HAVING NUMBER REVERSE TERMINATE
IDENTIFIED NUMBER_BASE REVOKE THEN
IF OF ROLLBACK TO
IN ON ROWID TRUE
INDEX OPEN ROWLABEL TYPE
INDEXES OPTION ROWNUM UNION
INDICATOR OR ROWTYPE UNIQUE
INSERT ORDER RUN UPDATE
INTEGER OTHERS SAVEPOINT USE
INTERSECT OUT SCHEMA VALUES
INTO PACKAGE SELECT VARCHAR
IS PARTITION SEPARATE VARCHAR2
LEVEL PCTFREE SET VARIANCE
LIKE POSITIVE SIZE VIEW
LIMITED PRAGMA SMALLINT VIEWS
LOOP PRIOR SPACE WHEN
MAX PRIVATE SQL WHERE
MIN PROCEDURE SQLCODE WHILE
MINUS PUBLIC SQLERRM WITH
MLSLABEL RAISE START WORK
MOD RANGE STATEMENT XOR
MODE REAL STDDEV
NATURAL RECORD SUBTYPE

 

除去保留词和关键词外,Oracle内部还存在一张命名空间(namespace)的列表,这些命名空间或为OCI函数或为Oracle内部函数作为函数的起始字符,如SQL*NET透明网络服务相关函数都以”NS”为函数名的开头,Oracle不推荐使用用户函数使用这些保留的命名空间为开头,以避免潜在的冲突可能。

Oracle Reserved Namespaces

Namespace Library
O OCI functions
S function names from SQLLIB and system-dependent libraries
XA external functions for XA applications only
GEN KP L NA NC ND NL NM NR NS NT NZ TTC UPI Internal functions

Latches and Tuning:The Library Cache

1. The shared pool is determined by the value of SHARED_POOL_SIZE as indicated in the init.ora file.  The library
cache is part of the shared pool.

a.  Determine the size of the shared poool by executing the following query:> select name,value from v$system_parameter where name = ‘shared_pool_size’;

The shared pool size is part of the VARIABLE SIZE value returned by querying V$SGA.

2.  The hidden parameter _KGL_BUCKET_COUNT determines the initial size of the hash table.  Typically, you
will accept the default value and not change this parameter.  The maximum value is 8.

a.  Determine the initial size of the hash table by executing the following query:> select ksppinm, ksppity from x$ksppi where ksppinm  = ‘_kgl_bucket_count’;

3. Object types are stored in a namespace.  While there can be 32 different namespaces, objects of the same
type will always be stored in the same namespace.

a.  Determine the namespaces allocated in the library cache by executing the following query:> select namespace from v$librarycache;

The number of namespaces are subject to increase at any time.  Common namespaces are:

CRSR: stores library cache objects of type cursor (shared SQL statements)
TABL/PRCD/TYPE: stores tables, views, sequences, synonyms, and procedure specifications
BODY/TYBD: stores procedure, function, package, and type bodies
INDX: stores librarcy cache objects of type index
TRGR: stores librarcy cache objects of type trigger
CLST: stores librarcy cache objects of type cluster

4.  Object tables are maintained for each object.  While the contents of each X$ table are fairly obscure, you can
query them to derive information about objects.

a.  Describe and/or query one of the following tables.X$KGLDP: (Dependency Table) One entry for each object that this object depends on.  For example, a view would depend on underlying tables or views.
X$KGLTR: (Translation Table) Contains records explaining how names referenced by this object were resolved to base objects.
X$KGLAU: (Authorization Table) Contains entries for each privilege defined on the object.
X$KGLXS: (Access Table) One or more entries for each entry in the dependency table.
X$KGLRD: (Read-only dependency table) Like the dependency table but for read only objects.
X$KGLSN: (Schema Name table) Only cursors have schema name tables and they store the schema names for the objects in the authorization table.
(Child Table) One entry for each object that is a child of this object.

5.  Remember that there are 32 namespaces in the library cache.  Also, each object has three types of flags; public,
status, and special status.
a.  Determine the number of namespaces by querying the OBJ$ table:

> select distinct(namespace) from obj$;

b. You can see the name of objects, their namespace, and flags by executing the following query.  Since there are many objects the query is limited using the psuedo column rownum.

> select name,namespace,flags from obj$ where flags > 1 and rownum < 10;

6. Locks and pins are used to control acccess to library cache objects. The X$KGLLK table, as indicated by the
letters LK, records locks on library cache objects.

a.  Connect as SYS and query the X$KGLLK table using the following query:> select user_name from x$kgllk where user_name = ‘SCOTT’;

Provided SCOTT is not logged in, this query should return no rows.:

b.  Create a second SQL*PLUS session as SCOTT/TIGER.

c.  Switch to SYS’s SQL*PLUS session and execute the following query:

> select user_name, kglnaobj from x$kgllk where user_name = ‘SCOTT’;

The user SCOTT acquired object handles on the objects DBMS_APPLICATION_INFO and DBMS_OUTPUT.

d. Switch to SCOTT’s SQL*PLUS session and execute the following update statement:

> update dept set dname = ‘TEST’ where deptno = ’10’;

e. Switch to SYS’s SQL*PLUS session and execute the following query:

> select user_name, kglnaobj from x$kgllk where user_name = ‘SCOTT’;

You will see that SCOTT has acquired additional locks as a result of the update statement.

7.  The X$KGLLPN table, as indicated by the letters PN, records pins on library cache objects.  The contents of the
X$KGLPN table are obscure but you may want to take a look at the data.
a.  Describe the X$KGLN table:

> desc X$KGLN

8. Library cache contention can be caused by excessive parsing of SQL statements.

a.  Determine the parse count in the library cache by executing the following query:b.  Create a second SQL*PLUS session as SCOTT/TIGER.c.  Switch to SYS’s SQL*PLUS session and execute the following query:

> select user_name, kglnaobj from x$kgllk where user_name = ‘SCOTT’;

9.  One diagnostic you can use for determing library cache performance is querying the V$LIBRARYCACHE
view.

a.  Execute the following query:> select namespace, gets, gethitratio, pins, pinhitratio, reloads, invalidations
from v$librarycache;

  • NAMESPACE: the different library cache namespaces
  • GETS: the total number of calls to locate and lock an object in the library cache
  • PINS: total number of calls to pin an object heap (to examine and possibly change)
  • GET/PINHITRATIO: ratio of overall requests to successful acquisitions for the GET and PIN calls in the cache
  • RELOADS: object reloads due to being aged out of the library cache
  • INVALIDATIONS: number of times the object was invalidated
  • Tuning Recommendations:

  • Keep the HITRATIOS above 90%
  • Keep the RELOADS to a minimum, ideally close to zero
  • Avoid DDL and minimize user role changes in a busy production environmentto prevent INVALIDATIONS
  • Size the shared pool appropriately so as to avoid objects getting aged out of the library cache
  • Similar SQL statements must be identical to be shared – use bind variables instead of literals
  • 10.  By performing a library cache dump you can gather extensive information about the library cache.  The dump will
    show you all of the namespaces, buckets, librarcy cache statistics, and content of the librarcy cache.  Beware, if you have
    a large database this dump file can be quite large and take a long time to generate.  You may want to select the
    appropriate level 1 – 4, depending upon the information you want to see.

    a.  As user SYS, execute the following query:> alter session set events ‘immediate trace name library_cache level 4’;

    The output will be generated in the USER_DUMP_DEST directory.

  • Level 1: dump libracy cache statistics
  • Level 2: include a hash table histogram; each bucket has one asterisk for each included handle
  • Level 3: include the dump of the object handles
  • Level 4: include the dump of the object structures (heap 0 only)
  • 11.  The X$KSMLRU fixed table tracks allocations in the shared pool that cause other objects to be aged out.  This
    table can be used to identify what is causing the large allocation.

    a.  Query the X$KSMLRU fixed table:> select * from x$ksmlru where ksmlru > 4000;

    The table contents are deleted after a SELECT.  This is done because the table stores only the largest allocations that have occurred.

    b.  Describe the X$KSMLRU table:

    > desc X$KSMLRU

    KSMLRSIZ: amount of contiguous memory being allocated.  Values over 5KB start to be a problem.
    KSMLRNUM: number of objects that were flushed from the shared pool in order to allocate the memory.
    KSMLRHON: the name of the object being loaded into the shared pool if the object is a PL/SQL object or a cursor.
    KSMLROHV: hash value of object being loaded.
    KSMLRSES: SADDR of the session that loaded the object.

    12.  One way to decrease the load on the library cache latch is to reduce the number of parse calls that are coming into
    the system.

    a.  To identify statements that are receiving a lot of parse calls, execute the following statement:> select sql_text, parse_calls, executions
    from v$sqlarea
    where parse_calls > 100
    and executions< 2 * parse_calls;

    13.  An additional method to tune the library cache is to convert anonymous blocks into packages if possible.
    a.  Find anonymous  blocks by executing the following query (47 is the command for an anonymous PL/SQL block):

    > select sql_text
    from v$sqlarea
    where command_type = 47;

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