1 Don’t Shut Down That Database! Use Oracle 9i Online Object Redefinition Instead Chris Lawson...

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Don’t Shut Down That Database!

Use Oracle 9i Online Object Redefinition Instead

Chris LawsonPerformance Solutions

Roger SchragDatabase Specialists, Inc.

IOUG Live! April, 2002Session #524

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Session Topics

What is “online object redefinition”? Why is this feature important? The dbms_redefinition package The online redefinition process Examples Restrictions Lessons learned

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Online Object Redefinition

Oracle 8i provided some online capabilities for IOTs.

Oracle 9i enables a wider range of online maintenance for almost all types of tables.

The ability to change the definition of a database object without restricting the ability of users to query and update data

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Things You Can Do With Online Redefinition

Move a table or index to a new tablespace Change a table’s organization (partitioning,

index-organized, etc.) Add, remove, or rename columns in a table Change the data type of a column in a table Add new indexes to a table Change constraint definitions on a table

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Why Do We Need This Feature?

Databases are getting larger and more complex.

DBAs are being given less down time to perform maintenance.

Companies often insist on no downtime. This feature makes it possible to perform

many important maintenance tasks online.

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The dbms_redefinition Package

CAN_REDEF_TABLE START_REDEF_TABLE FINISH_REDEF_TABLE ABORT_REDEF_TABLE SYNC_INTERIM_TABLE

Use the five procedures in this package to redefine an object online.

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Permissions Required To Redefine A Table Online EXECUTE on the dbms_redefinition package CREATE ANY TABLE ALTER ANY TABLE DROP ANY TABLE LOCK ANY TABLE SELECT ANY TABLE

Typically, you’ll want to use a DBA account to redefine objects online.

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Overview Of The Online Redefinition Process

Step 1: Verify the table is eligible for online redefinition.

Step 2: Create an interim table.Step 3: Associate the interim table with the

table to be redefined.Step 4: Add constraints, indexes, triggers, and

grants to the interim table.Step 5: Complete the online redefinition.

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Step 1: Verify The Table Is Eligible For Online Redefinition

Confirms table to be redefined meets requirements for online object redefinition

Call the CAN_REDEF_TABLE procedure in the dbms_redefinition package:EXECUTE dbms_redefinition.can_redef_table ('FINANCE', 'RATE');

Table qualifies if no exception is raised Exception will be raised if table does not qualify:

ORA‑12089: Cannot online redefine table with no primary key

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Step 2: Create An Interim Table

Online redefinition uses an interim table to change the object definition without restricting user access

Create interim table using exact column names, data types, organization, tablespace assignment, and storage clause that the redefined table should have

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Declare primary key on interim table, but do not create any other indexes, constraints, or triggers

Oracle will copy rows from the original table into the interim table during the redefinition process

Oracle will swap the names of the two tables at the end so the interim table will become the production table and vice versa

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Step 3: Associate The Interim Table With The Table To Be

Redefined Call the START_REDEF_TABLE procedure:

EXECUTE dbms_redefinition.start_redef_table ('FINANCE', 'RATE', 'STAGING');

Oracle creates a materialized view and log in order to populate the interim table from the production table and track subsequent updates to the production table. All rows in the production table are copied to the interim table

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Redefined A column mapping must be specified when calling

START_REDEF_TABLE if any columns are being added or removed, if any data types are being changed, or if any data is being modified

Could take a long time if production table has many rows

Users have full query and update capability

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Step 4: Add Constraints, Indexes, Triggers, And Grants To Interim

Table Create constraints, indexes, database triggers,

and grants on interim table desired on the production table at the end of the online redefinition process

Features on the production table, but not the interim table, will disappear from the redefined table in the next step

Create foreign keys with the DISABLE keyword

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Step 5: Complete The Online Redefinition

Interim table has organization, column definitions, indexes, constraints, triggers, and grants desired on redefined table. Interim table also has all of the rows of the production table except for updates occurring since Step 3 began

Call the FINISH_REDEF_TABLE procedure:

EXECUTE dbms_redefinition.finish_redef_table ('FINANCE', 'RATE', 'STAGING');

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Oracle uses the materialized view log to propagate pending updates to interim table

Oracle locks production and interim tables briefly and swaps their names in data dictionary

Oracle drops the materialized view and log and enables disabled foreign keys

Redefinition is complete. Interim table (which used to be the production table) may be dropped

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Step usually runs quickly (unless users have updated a lot of rows in the production table between the time Step 3 began and the time this step began)

Tables are locked only briefly. At all other times during the entire process, users have complete query and update ability on the table undergoing online redefinition

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Aborting An Online Redefinition

You may cancel an online redefinition process at any time before calling the FINISH_REDEF_TABLE procedure.

Abort the online redefinition by calling the ABORT_TABLE_REDEF procedure. Oracle drops the materialized view and log definitions, if they had been created.

You may drop the interim table if you wish.

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Examples Of Online Object Redefinition

1. Move a table to a new tablespace and add two new indexes.

2. Massage values in one column of a table and change the data type of another column.

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Example #1: Move A Table To A New Tablespace And Add Two

New Indexes

ALTER TABLE...MOVE locks the table. Export / Import makes the table read-only during

export and inaccessible during import. CREATE INDEX locks the table. Online redefinition allows full query and update

access.

Move the accts_payable table in the finance schema to a new tablespace. At the same time, add indexes on the vendor_name and vendor_po columns.

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Step 1: Verify The Table Is Eligible For Online

RedefinitionSQL> BEGIN 2 dbms_redefinition.can_redef_table 3 ('FINANCE', 'ACCTS_PAYABLE'); 4 END; 5 / PL/SQL procedure successfully completed SQL>

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SQL> CREATE TABLE accts_payable_stage 2 ( 3 account_id VARCHAR2(20), 4 vendor_name VARCHAR2(50) NOT NULL, 5 vendor_address VARCHAR2(50), 6 vendor_po VARCHAR2(20), 7 invoice_date DATE NOT NULL, 8 invoice_amount NUMBER NOT NULL, 9 CONSTRAINT accts_payable_pk PRIMARY KEY (account_id) 10 USING INDEX TABLESPACE accts_pay_ind 11 ) 12 TABLESPACE accts_pay_tab; Table created. SQL>

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Same column names and data types as production table

Primary key declared, but no other indexes or constraints

Desired tablespaces, storage clauses, and organization specified

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RedefinedSQL> BEGIN 2 dbms_redefinition.start_redef_table 3 ('FINANCE', 'ACCTS_PAYABLE', 4 'ACCTS_PAYABLE_STAGE'); 5 END; 6 / PL/SQL procedure successfully completed. SQL>

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Redefined No column mapping provided because no column information changed in this example

Step could take a long time if accts_payable table has many rows

Users will have full query and update ability

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Step 4: Add Constraints, Indexes, Triggers, And Grants

To Interim TableSQL> CREATE INDEX accts_payable_n1 ON accts_payable_stage (vendor_name) 2 TABLESPACE accts_pay_ind; Index created. SQL> CREATE INDEX accts_payable_n2 ON accts_payable_stage (vendor_po) 2 TABLESPACE accts_pay_ind; Index created. SQL> CREATE INDEX accts_payable_n3 ON accts_payable_stage (invoice_date) 2 TABLESPACE accts_pay_ind; Index created. SQL>

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To Interim Table Add to interim table whatever features are

desired on redefined table Could be quite different from what exists on

original table (new indexes, different constraints, etc.)

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SQL> BEGIN

2 dbms_redefinition.finish_redef_table

3 ('FINANCE', 'ACCTS_PAYABLE',

4 'ACCTS_PAYABLE_STAGE');

5 END;

6 /

PL/SQL procedure successfully completed.

SQL>

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Could take a long time if many updates occurred after Step 3 began

Production table locked briefly near end of step

Production and interim table names are swapped

Drop interim table (old production table) manually after online redefinition is complete

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Example #2: Massage Values In One Column Of A Table And

Change The Data Type Of Another Column

SQL> DESCRIBE invoices Name Null? Type ---------------- -------- ---- INVOICE_ID NOT NULL VARCHAR2(20) LINE_ITEM NOT NULL NUMBER DESCRIPTION VARCHAR2(200) VENDOR_ID NOT NULL NUMBER INVOICE_AMT NOT NULL NUMBER

Strip trailing blanks from all values in the description column of the invoices table. Also, change the data type of the vendor_id column.

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Step 1: Verify The Table Is Eligible For Online

RedefinitionSQL> BEGIN 2 dbms_redefinition.can_redef_table 3 ('FINANCE', 'INVOICES'); 4 END; 5 / PL/SQL procedure successfully completed SQL>

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SQL> CREATE TABLE invoices_stage 2 ( 3 invoice_id VARCHAR2(20), 4 line_item NUMBER NOT NULL, 5 description VARCHAR2(200), 6 vendor_id VARCHAR2(20) NOT NULL, 7 invoice_amt NUMBER NOT NULL, 8 CONSTRAINT invoices_pk PRIMARY KEY (invoice_id) 9 USING INDEX TABLESPACE ind 10 ) 11 TABLESPACE tab; Table created. SQL>

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Data type of vendor_id column has changed from NUMBER to VARCHAR2(20)

Tablespaces specified even though we are not moving table or index to new tablespace

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RedefinedSQL> BEGIN 2 dbms_redefinition.start_redef_table 3 ('FINANCE', 'INVOICES', 'INVOICES_STAGE', 4 'INVOICE_ID INVOICE_ID, LINE_ITEM LINE_ITEM, ' || 5 'RTRIM (DESCRIPTION) DESCRIPTION, ' || 6 'TO_CHAR (VENDOR_ID) VENDOR_ID, ' || 7 'INVOICE_AMT INVOICE_AMT'); 8 END; 9 / PL/SQL procedure successfully completed. SQL>

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Redefined Column mapping provided due to data massaging and type conversion

RTRIM function to strip trailing blanks from description column

TO_CHAR function to convert numeric vendor_id to VARCHAR2

Invoice descriptions and vendor IDs are transformed as rows are copied to interim table

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To The Interim Table For simplicity, the invoices table in this

example does not have any constraints, indexes, triggers, or grants other than the primary key declared when the interim table was created.

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SQL> BEGIN

2 dbms_redefinition.finish_redef_table

3 ('FINANCE', 'INVOICES', 'INVOICES_STAGE');

4 END;

5 /

PL/SQL procedure successfully completed.

SQL>

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At the end of the online redefinition, the description column is free of trailing blanks and the data type of the vendor_id column has been changed:

SQL> DESCRIBE invoices Name Null? Type ---------------- -------- ---- INVOICE_ID NOT NULL VARCHAR2(20) LINE_ITEM NOT NULL NUMBER DESCRIPTION VARCHAR2(200) VENDOR_ID NOT NULL VARCHAR2(20) INVOICE_AMT NOT NULL NUMBER SQL>

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Limitations Of The Online Object Redefinition Feature

Table eligibility requirements Limitations of column mapping Row selectivity

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Not Eligible For Online Redefinition

Tables without a declared primary key Tables that are the basis of a materialized

view Tables belonging to a cluster Temporary tables Tables in the SYS or SYSTEM schema Tables with FILE, LONG, or user-defined

data types

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Column Mapping Limitations

Simple functions and expressions only– Allowed: Changing the case of a string of text or

multiplying a numeric value by a constant– Not allowed: Subqueries or non-deterministic

functions

Mandatory columns must get values derived from existing columns

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Row Selectivity Limitation

Cannot eliminate rows from a table during online redefinition

Redefined table will have all of the rows in the original table

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Lessons Learned

Testing with the interim table Constraint validation Handling foreign keys Column mapping Speeding up the FINISH_REDEF_TABLE

call

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Testing With The Interim Table

Verify column mapping Validate data transformation Test new indexes Abort the redefinition if problems are

discovered—without impacting production

You can validate your object redefinition against the interim table before calling the FINISH_REDEF_TABLE procedure.

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Constraint Validation

Foreign keys declared on the redefined table are enabled but not validated at end of redefinition

Existing rows not tested for compliance Validating existing rows must be done

manually if validation desired

The Oracle 9i documentation fails to mention this point!

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Handling Foreign Keys

Recall that at the end of an online object redefinition, the names of the original production table and the interim table are swapped.

This means that after an online redefinition, foreign keys on other tables that used to reference the production table will now reference the interim table.

Redefining a table that is a parent to foreign keys declared on other tables presents a special problem.

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Redefining A Table That Is A Parent To Foreign Keys Declared

On Other Tables Declare new foreign keys in disabled state

that reference the interim table. When redefinition is complete, drop foreign

keys that reference the interim table and enable foreign keys that reference the redefined table.

Plan for locks and resources required to enable the new constraints.

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Column Mapping Specify column map in call to

START_REDEF_TABLE if any column names, data types, or values are changed

Single string consisting of comma-delimited expression / name pairs– An expression is a formula to derive column

values from the existing production table.– A name is the name of a column in the interim

table to be loaded with the value.

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SQL Syntax Refresher

Strings are delimited by single quotes. Include a single quote in a string by

specifying two single quotes in a row. Concatenate two strings with the || operator.

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Column Mapping ExampleAppend an “A” to each invoice number while redefining the invoices table.

SQL> BEGIN 2 dbms_redefinition.start_redef_table 3 ('FINANCE', 'INVOICES', 'INVOICES_INTERIM', 4 'INVOICE_ID INVOICE_ID, VENDOR_ID VENDOR_ID, ' || 5 'INVOICE_DATE INVOICE_DATE, AMOUNT AMOUNT, ' || 6 'INVOICE_NUMBER || ''A'' INVOICE_NUMBER'); 7 END; 8 / PL/SQL procedure successfully completed. SQL>

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Speeding Up The FINISH_REDEF_TABLE Call Materialized view log catches all updates to

production table after START_REDEF_TABLE call All pending changes are propagated to interim

table during FINISH_REDEF_TABLE call Can call SYNC_INTERIM_TABLE before

FINISH_REDEF_TABLE to propagate all pending changes

Does not speed up the overall redefinition process

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Wrapping Up Oracle Corporation recognizes the need for high

availability. By and large, the online redefinition feature of

Oracle 9i is pretty easy to use. Tricky issues, such as foreign key handling, stem

from the complexities of changing a table while users are querying and updating it.

As with any new Oracle feature, test thoroughly before using!

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Contact InformationChris LawsonPerformance SolutionsTel: 925/829-7496Email: [email protected] Web: www.oraclemagician.com

Roger SchragDatabase Specialists, Inc.Tel: 415/344-0500Email: [email protected]: www.dbspecialists.com

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