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The Oracle Data Dictionary

 

• Data Dictionary is stored in the SYSTEM tablespace. Owner is SYS.
• Some data dictionary information is kept in the SGA dictionary cache.
• Main tables are:
• obj$,tab$,ind$,undo$,ts$,seg$,user$,dependency$
• Data dictionary objects that cannot be altered with a DDL, are stored in table Bootstrap$. (Error ORA-701)

Bootstrap$ table 
SQL> desc sys.bootstrap$
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 LINE#                                     NOT NULL NUMBER
 OBJ#                                      NOT NULL NUMBER
 SQL_TEXT                               NOT NULL VARCHAR2(4000)





Basic Relationship





How to Detect - Dictionary corruptions ?

• Hcheck8i.sql – Note:136697.1

 

Common Data Dictionary Corruptions

• Missing rows in tables. Example:
  • Tab$/Ind$ with no entries in obj$.
  • Undo$/Tab$/Ind$ with no entries in seg$.
  • Obj$ has no entries in user$ (owner#).
  • Obj$ with no entries in tab$,ind$,seq$,etc.
• Missing data dictionary objects.
• Corrupted data dictionary objects (tables or indexes):
  • Analyze validate structure cascade fails with ORA-1499 (table/index cross reference failure – see trace file)

 

• Invalid entries in data dictionary tables.

 

Understanding Bootstrap Of Oracle Database

Applies to:

Oracle Server – Enterprise Edition – Version:
Information in this document applies to any platform.
Oracle RDBMS Server

Purpose

What is bootstrap?
What happens during database startup?
Which objects are classified as bootstrap objects in oracle database?
How the dictionaries are loaded?
Why bootstrap (ORA-00704) process failure is so serious if it occurs on startup or while recovery?

Scope and Application

This article intend to explain the bootstrap operations of oracle.

Understanding Bootstrap Of Oracle Database

What is bootstrap?

In general, Bootstrap is a technique for loading the first few instructions of a computer program into active memory and then using them to bring in the rest of the program.

What is bootstrap in Oracle ?

In Oracle, Bootstrap refers to loading of metadata (data dictionary) before we OPEN the database.
Bootstrap objects are classified as the objects (tables / indexes / clusters) with the object_id below 56 as bootstrap objects.
These objects are mandatory to bring up an instance, as this contains the most important metadata of the database.

What happens on database startup?

This shall be explained by setting the SQL_TRACE while opening the database.

Connect as sysdba and do the following

STARTUP MOUNT; ALTER SESSION SET EVENTS ’10046 TRACE NAME CONTEXT FOREVER, LEVEL 12′; ALTER DATABASE OPEN; ALTER SESSION SET EVENTS ’10046 TRACE NAME CONTEXT OFF’; SHOW PARAMETER USER_DUMP_DEST ORADEBUG SETMYPID ORADEBUG TRACEFILE_NAME

The sql_trace of the above process explains the following operations behind startup.
The bootstrap operation happens between MOUNT stage and OPEN stage.

1. The first SQL after in the above trace shows the creation of the bootstrap$ table.
Something similar to the following:

———————————————–
create table bootstrap$ ( line# number not null, obj# number not null, sql_text varchar2(4000) not null) storage (initial 50K objno 56 extents (file 1 block 377))
———————————————–

This sys.bootstrap$ table contains the DDL’s for other bootstrap tables (object_id below 56).

-> Actually these tables were created internally by the time of database creation (by sql.bsq), The create DDL passed between MOUNT and OPEN stage will be executed through different driver routines. In simple words these are not standard CREATE DDLs.
While starting up the database oracle will load these objects into memory (shared_pool),
(ie) it will assign the relevant object number and refer to the datafile and the block associated with that.
And such operations happen only while warm startup.

@ The internals of the above explained in ‘kqlb.c’.

2. Now a query executed against the sys.bootstrap$ table, which holds the create sql’s for other base tables.

select line#, sql_text from bootstrap$ where obj# != :1 (56)

Subsequently it will create those objects by running those queries.

Object number 0 – (System Rollback Segment)
Object number 2 to 55 (Other base tables)
Object number 1 is NOT used by any of the objects.

3. Performs various operations to keep the bootstrap objects in consistent state.

– Upon the successful completion of bootstrap the database will do the other tasks like recovery and will open the database.

Which objects are classified as bootstrap objects in oracle database?

Objects with data_object_id less than 56 are classified as core bootstrap objects.

The objects are added to the bootstrap. The objects affected are:
hist_head$
histgrm$
i_hh_obj#_col#
i_hh_obj#_intcol#
i_obj#_intcol#
i_h_obj#_col#
c_obj#_intcol#

From 10.1 the following objects have been added:
fixed_obj$
tab_stats$
ind_stats$
i_fixed_obj$_obj#
i_tab_stats$_obj#
i_ind_stats$_obj#
object_usage

These additional objects shall be re-classified (or) ignored by following methods.
1. Opening the database in migrate mode
2. Using event 38003
Event 38003 affects the bootstrap process of loading the fixed cache in  kqlblfc(). Per default certain objects are marked as bootstrap objects (even though they are not defined as such in sys.bootstrap$) but by setting the event they will be left as non-bootstrapped.

What is bootstrap process failure? ORA-00704

This ORA-00704 error SERIOUS if reported at startup. This error refers to some problem during bootstrap operation.
Any ORA-00704 error on STARTUP / RECOVER is serious, this error normally rose due to some inconsistency with the bootstrap segments (or) data corruption on bootstrap$ (or) any of the base tables below object_id 56. After this error it might not allow to open that database.

When ORA-00704 shall occur?

1. There is a probable of this error when any unsupported operations are tried to force open the database.
2. This error can also occur when system datafile has corrupted blocks. (ORA-01578)
3. In earlier releases of oracle (prior to 7.3.4 and 8.0.3) this issue shall arise due to Bug 434596

The option is to restore it from a good backup and recover it.
-> If the underlying cause is physical corruption that is due to hardware problems then do complete recovery.
-> If the issue is not relating to any physical corruption, then the problem could be due some unsupported actions on Bootstrap, and a Point In Time Recovery would be an option in such cas