Summary Prerequisites and limitations Architecture Tools Epics Related resources Additional information

Migrate Oracle ROWID functionality to PostgreSQL on AWS

Created by Rakesh Raghav (AWS) and Ramesh Pathuri (AWS)

Environment: PoC or pilot	Source: Oracle Database	Target: PostgreSQL database on AWS
R Type: Replatform	Workload: Oracle	Technologies: Migration; Databases
AWS services: Amazon Aurora; Amazon RDS; AWS SCT; AWS CLI

Summary

This pattern describes options for migrating the ROWID pseudocolumn functionality in Oracle Database to a PostgreSQL database in Amazon Relational Database Service (Amazon RDS) for PostgreSQL, Amazon Aurora PostgreSQL-Compatible Edition, or Amazon Elastic Compute Cloud (Amazon EC2).

In an Oracle database, the ROWID pseudocolumn is a physical address of a row in a table. This pseudocolumn is used to uniquely identify a row even if the primary key isn’t present on a table. PostgreSQL has a similar pseudocolumn called ctid, but it cannot be used as a ROWID. As explained in the PostgreSQL documentation, ctid might change if it’s updated or after every VACUUM process.

There are three ways you can create the ROWID pseudocolumn functionality in PostgreSQL:

Use a primary key column instead of ROWID to identify a row in a table.
Use a logical primary/unique key (which might be a composite key) in the table.
Add a column with auto-generated values and make it a primary/unique key to mimic ROWID.

This pattern walks you through all three implementations and describes the advantages and disadvantages of each option.

Prerequisites and limitations

Prerequisites

An active AWS account
Procedural Language/PostgreSQL (PL/pgSQL) coding expertise
Source Oracle Database
An Amazon RDS for PostgreSQL or Aurora PostgreSQL-Compatible cluster, or an EC2 instance to host the PostgreSQL database

Limitations

This pattern provides workarounds for the ROWID functionality. PostgreSQL doesn’t provide an equivalent to ROWID in Oracle Database.

Product versions

PostgreSQL 11.9 or later

Architecture

Source technology stack

Oracle Database

Target technology stack

Aurora PostgreSQL-Compatible, Amazon RDS for PostgreSQL, or an EC2 instance with a PostgreSQL database

Converting an Oracle Database to PostgreSQL on AWS

Implementation options

There are three options to work around the lack of ROWID support in PostgreSQL, depending on whether your table has a primary key or unique index, a logical primary key, or an identity attribute. Your choice depends on your project timelines, your current migration phase, and dependencies on application and database code.

Option	Description	Advantages	Disadvantages
Primary key or unique index	If your Oracle table has a primary key, you can use the attributes of this key to uniquely identify a row.	No dependency on proprietary database features. Minimal impact on performance, because primary key fields are indexed.	Requires changes to application and database code that relies on `ROWID` to switch to primary key fields.
Logical primary/unique key	If your Oracle table has a logical primary key, you can use the attributes of this key to uniquely identify a row. A logical primary key consists of an attribute or a set of attributes that can uniquely identify a row, but it isn’t enforced on the database through a constraint.	No dependency on proprietary database features.	Requires changes to application and database code that relies on `ROWID` to switch to primary key fields. Significant impact on performance if the attributes of the logical primary key aren’t indexed. However, you can add a unique index to prevent performance issues.
Identity attribute	if your Oracle table doesn't have a primary key, you can create an additional field as `GENERATED ALWAYS AS IDENTITY`. This attribute generates a unique value whenever data is inserted into the table, so it can be used to uniquely identify a row for Data Manipulation Language (DML) operations.	No dependency on proprietary database features. PostgreSQL database populates the attribute and maintains its uniqueness.	Requires changes to application and database code that relies on `ROWID` to switch to identity attribute. Significant impact on performance if the additional field isn’t indexed. However, you can add an index to prevent performance issues.

Tools

Amazon Relational Database Service (Amazon RDS) for PostgreSQL helps you set up, operate, and scale a PostgreSQL relational database in the AWS Cloud.
Amazon Aurora PostgreSQL-Compatible Edition is a fully managed, ACID-compliant relational database engine that helps you set up, operate, and scale PostgreSQL deployments.
AWS Command Line Interface (AWS CLI) is an open-source tool that helps you interact with AWS services through commands in your command-line shell. In this pattern, you can use the AWS CLI to run SQL commands through pgAdmin.
pgAdmin is an open-source management tool for PostgreSQL. It provides a graphical interface that helps you create, maintain, and use database objects.
AWS Schema Conversion Tool (AWS SCT) supports heterogeneous database migrations by automatically converting the source database schema and a majority of the custom code to a format that’s compatible with the target database.

Epics

Task Description Skills required

Task	Description	Skills required
Identify Oracle tables that use the `ROWID` attribute.	Use the AWS Schema Conversion Tool (AWS SCT) to identify Oracle tables that have `ROWID` functionality. For more information, see the AWS SCT documentation. —or— In Oracle, use the `DBA_TAB_COLUMNS` view to identify tables that have a `ROWID` attribute. These fields might be used to store alphanumeric 10-byte characters. Determine the usage and convert these to a `VARCHAR` field if appropriate.	DBA or developer
Identify code that references these tables.	Use AWS SCT to generate a migration assessment report to identify procedures affected by `ROWID`. For more information, see the AWS SCT documentation. —or— In the source Oracle database, use the text field of the `dba_source` table to identify objects that use `ROWID` functionality.	DBA or developer

Identify Oracle tables that use the ROWID attribute.

Use the AWS Schema Conversion Tool (AWS SCT) to identify Oracle tables that have ROWID functionality. For more information, see the AWS SCT documentation.

—or—

In Oracle, use the DBA_TAB_COLUMNS view to identify tables that have a ROWID attribute. These fields might be used to store alphanumeric 10-byte characters. Determine the usage and convert these to a VARCHAR field if appropriate.

DBA or developer

Identify code that references these tables.

Use AWS SCT to generate a migration assessment report to identify procedures affected by ROWID. For more information, see the AWS SCT documentation.

—or—

In the source Oracle database, use the text field of the dba_source table to identify objects that use ROWID functionality.

DBA or developer

Task Description Skills required

Task	Description	Skills required
Identify tables that don’t have primary keys.	In the source Oracle database, use `DBA_CONSTRAINTS` to identify tables that don’t have primary keys. This information will help you determine the strategy for each table. For example: `select dt.* from dba_tables dt where not exists (select 1 from all_constraints ct where ct.owner = Dt.owner and ct.table_name = Dt.table_name and ct.constraint_type = 'P' ) and dt.owner = '{schema}'`	DBA or developer

Identify tables that don’t have primary keys.

In the source Oracle database, use DBA_CONSTRAINTS to identify tables that don’t have primary keys. This information will help you determine the strategy for each table. For example:


select dt.*
from dba_tables dt
where not exists (select 1
                  from all_constraints ct
                  where ct.owner = Dt.owner
                    and ct.table_name = Dt.table_name
                    and ct.constraint_type = 'P'
                  )
and dt.owner = '{schema}'

DBA or developer

Task	Description	Skills required
Apply changes for tables that have a defined or logical primary key.	Make the application and database code changes shown in the Additional information section to use a unique primary key or a logical primary key to identify a row in your table.	DBA or developer
Add an additional field to tables that don’t have a defined or logical primary key.	Add an attribute of type `GENERATED ALWAYS AS IDENTITY`. Make the application and database code changes shown in the Additional information section.	DBA or developer
Add an index if necessary.	Add an index to the additional field or logical primary key to improve SQL performance.	DBA or developer

Related resources

PostgreSQL CTID (PostgreSQL documentation)
Generated Columns (PostgreSQL documentation)
ROWID Pseudocolumn (Oracle documentation)

Additional information

The following sections provide Oracle and PostgreSQL code examples to illustrate the three approaches.

Scenario 1: Using a primary unique key

In the following examples, you create the table testrowid_s1 with emp_id as the primary key.

Oracle code:


create table testrowid_s1 (emp_id integer, name varchar2(10), CONSTRAINT testrowid_pk PRIMARY KEY (emp_id));
INSERT INTO testrowid_s1(emp_id,name) values (1,'empname1');
INSERT INTO testrowid_s1(emp_id,name) values (2,'empname2');
INSERT INTO testrowid_s1(emp_id,name) values (3,'empname3');
INSERT INTO testrowid_s1(emp_id,name) values (4,'empname4');
commit;

SELECT rowid,emp_id,name FROM testrowid_s1;
ROWID                  EMP_ID NAME
------------------ ---------- ----------
AAAF3pAAAAAAAMOAAA          1 empname1
AAAF3pAAAAAAAMOAAB          2 empname2
AAAF3pAAAAAAAMOAAC          3 empname3
AAAF3pAAAAAAAMOAAD          4 empname4

UPDATE testrowid_s1 SET name = 'Ramesh' WHERE rowid = 'AAAF3pAAAAAAAMOAAB' ;
commit;

SELECT rowid,emp_id,name FROM testrowid_s1;
ROWID                  EMP_ID NAME
------------------ ---------- ----------
AAAF3pAAAAAAAMOAAA          1 empname1
AAAF3pAAAAAAAMOAAB          2 Ramesh
AAAF3pAAAAAAAMOAAC          3 empname3
AAAF3pAAAAAAAMOAAD          4 empname4

PostgreSQL code:


CREATE TABLE public.testrowid_s1
(
    emp_id integer,
    name character varying,
    primary key (emp_id)
);

insert into public.testrowid_s1 (emp_id,name) values 
(1,'empname1'),(2,'empname2'),(3,'empname3'),(4,'empname4');

select emp_id,name from testrowid_s1;
 emp_id |   name   
--------+----------
      1 | empname1
      2 | empname2
      3 | empname3
      4 | empname4

update testrowid_s1 set name = 'Ramesh' where emp_id = 2 ;

select emp_id,name from testrowid_s1;
 emp_id |   name   
--------+----------
      1 | empname1
      3 | empname3
      4 | empname4
      2 | Ramesh

Scenario 2: Using a logical primary key

In the following examples, you create the table testrowid_s2 with emp_id as the logical primary key.

Oracle code:


create table testrowid_s2 (emp_id integer, name varchar2(10) );
INSERT INTO testrowid_s2(emp_id,name) values (1,'empname1');
INSERT INTO testrowid_s2(emp_id,name) values (2,'empname2');
INSERT INTO testrowid_s2(emp_id,name) values (3,'empname3');
INSERT INTO testrowid_s2(emp_id,name) values (4,'empname4');
commit;

SELECT rowid,emp_id,name FROM testrowid_s2;
ROWID                  EMP_ID NAME
------------------ ---------- ----------
AAAF3rAAAAAAAMeAAA          1 empname1
AAAF3rAAAAAAAMeAAB          2 empname2
AAAF3rAAAAAAAMeAAC          3 empname3
AAAF3rAAAAAAAMeAAD          4 empname4

UPDATE testrowid_s2 SET name = 'Ramesh' WHERE rowid = 'AAAF3rAAAAAAAMeAAB' ;
commit;

SELECT rowid,emp_id,name FROM testrowid_s2;
ROWID                  EMP_ID NAME
------------------ ---------- ----------
AAAF3rAAAAAAAMeAAA          1 empname1
AAAF3rAAAAAAAMeAAB          2 Ramesh
AAAF3rAAAAAAAMeAAC          3 empname3
AAAF3rAAAAAAAMeAAD          4 empname4

PostgreSQL code:


CREATE TABLE public.testrowid_s2
(
    emp_id integer,
    name character varying
);

insert into public.testrowid_s2 (emp_id,name) values 
(1,'empname1'),(2,'empname2'),(3,'empname3'),(4,'empname4');

select emp_id,name from testrowid_s2;
 emp_id |   name   
--------+----------
      1 | empname1
      2 | empname2
      3 | empname3
      4 | empname4

update testrowid_s2 set name = 'Ramesh' where emp_id = 2 ;

select emp_id,name from testrowid_s2;
 emp_id |   name   
--------+----------
      1 | empname1
      3 | empname3
      4 | empname4
      2 | Ramesh

Scenario 3: Using an identity attribute

In the following examples, you create the table testrowid_s3 with no primary key and by using an identity attribute.

Oracle code:


create table testrowid_s3 (name varchar2(10));
INSERT INTO testrowid_s3(name) values ('empname1');
INSERT INTO testrowid_s3(name) values ('empname2');
INSERT INTO testrowid_s3(name) values ('empname3');
INSERT INTO testrowid_s3(name) values ('empname4');
commit;

SELECT rowid,name FROM testrowid_s3;
ROWID              NAME
------------------ ----------
AAAF3sAAAAAAAMmAAA empname1
AAAF3sAAAAAAAMmAAB empname2
AAAF3sAAAAAAAMmAAC empname3
AAAF3sAAAAAAAMmAAD empname4

UPDATE testrowid_s3 SET name = 'Ramesh' WHERE rowid = 'AAAF3sAAAAAAAMmAAB' ;
commit;

SELECT rowid,name FROM testrowid_s3;
ROWID              NAME
------------------ ----------
AAAF3sAAAAAAAMmAAA empname1
AAAF3sAAAAAAAMmAAB Ramesh
AAAF3sAAAAAAAMmAAC empname3
AAAF3sAAAAAAAMmAAD empname4

PostgreSQL code:


CREATE TABLE public.testrowid_s3
(
    rowid_seq bigint generated always as identity,
    name character varying
);

insert into public.testrowid_s3 (name) values 
('empname1'),('empname2'),('empname3'),('empname4');

select rowid_seq,name from testrowid_s3;
 rowid_seq |   name   
-----------+----------
         1 | empname1
         2 | empname2
         3 | empname3
         4 | empname4

update testrowid_s3 set name = 'Ramesh' where rowid_seq = 2 ;

select rowid_seq,name from testrowid_s3;
 rowid_seq |   name   
-----------+----------
         1 | empname1
         3 | empname3
         4 | empname4
         2 | Ramesh

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