The document discusses the challenges and strategies for migrating from Oracle to PostgreSQL, focusing on design, architecture, data migration, and application impacts. It outlines differences in database objects, data types, and the need for application code changes, as well as ways to optimize migration efforts. Additionally, it highlights PostgreSQL capabilities, including replication features and performance improvements.

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I am..
Venkata B Nagothi
Call me..
VENKAT

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Agenda
 Migration Challenges (Oracle to PostgreSQL)
 PostgreSQL capabilities

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Migration Challenges
 Design and Architecture challenges
 Development effort required for successful
database migration to PostgreSQL
 Minimize development effort when migrating
Applications

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Migration Assessment
Infrastructure
Database Objects
Data Migration
PL/SQL Objects
Database Environment
Application

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Infrastructure Migration

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High Availability
Oracle
Dataguard Broker
PostgreSQL
Streaming Replication
Migration Impact
• More simpler to implement
• Supports all the protection modes in Oracle
• Supports cascading replication with some limitations

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High Availability
Oracle
Dataguard Observer
PostgreSQL
pgPool-II
Master Standby
Switches over
when the
master crashes
pgPool-II
Automatically
promotes the
standby to
be the new
master
pgPool-II Automatic failover
disable
Application

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High Availability
Role reversal – Limitations in PostgreSQL
Master
Standby
(new master)
Streaming Replication
Standby can be
promoted to
Standalone
new master
manually
Old master cannot
be made standby.
Needs to rebuilt
from new master’s
backup

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High Availability
PostgreSQL Streaming Replication – Limitations
 Filesystem structure has to be identical
v
Master
Streaming Replication
/data/tbs01
/data/tbs02
/data/tbs03
Standby
/data/tbs01
/data/tbs02
/data/tbs03

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Tablespaces
Oracle Tablespaces
PostgreSQL
Tablespaces
Migration Impact
• More OS dependent
• Re-Architect database disk architecture
• Performance testing and benchmarking
• Re-do Capacity Planning

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Tablespaces
/disk1/datafile-1
/disk2/datafile-2
/disk3/datafile-3
Logical
Structure
Logical
Structure
Physical
Structure
Physical
Structure
/disk1/tbs01
Oracle Tablespace Structure PostgreSQL Tablespace Structure
Table 1 Table 2
Index 1
Table 1 Table 2
Index 1
Datafile 1 Datafile 2
Datafile 3

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Tablespaces
PostgreSQL Disk Architecture
/disk1/tbs01
/disk2/tbs02
/disk3/tbs03
/disk4/tbs04
/disk5/tbs05
Each partition has to be
an extendable partition for
better capacity management
A single partition over
multiple disk spindles
(RAID)
/data/pgdata

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Tablespaces
Limitations - PostgreSQL
 Tablespaces alone cannot be backed up
 Transportable tablespaces not possible
 Tablespace restore/recovery is not quite possible

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Databases
DB01
Instance 1
DB02
Instance 2
DB03
Instance 3
DB01
Instance 1
DB02
DB03
Oracle PostgreSQL
• Database Size
• Database performance
• Application
Architecture/behaviour
• Database operations
• Security
• Multitenancy
• Other business and
operational
constraints

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Database Objects Migration

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Schemas
Oracle Users
PostgreSQL
Users & Schemas
Migration Impact
• Users and Schemas must be separately created in PostgreSQL
• A user can own multiple schemas
• Flexibility around changing ownerships

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Database Objects
Oracle Objects PostgreSQL Objects
Materialized Views Materialized Views (limitations)
Index-Organized-Tables Cluster a Table (partial support)
Public Synonyms search_path
Global Temporary Tables Unlogged Tables

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Data Types
Oracle Data type PostgreSQL Data Type
VARCHAR2 VARCHAR
NUMBER INTEGER
DATE TIMESTAMP
CLOB TEXT / JSON / JSONB
BLOB BYTEA / JSONB
LO

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Data Migration

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Data Migration
BLOB
BYTEA
(1 GB LIMIT)
LO
BLOB JSONB

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Data Migration
Oracle
BLOB
PostgreSQL
BYTEA
(max limit 1 GB)
Ora2pg (automatic)
Migration Impact
• Oracle BLOBs are automatically migrated to PostgreSQL bytea by
ora2pg
• Unstable Application performance can be expected which would hit the
benchmarking metrics
• Can accommodate more than 1 GB due to PostgreSQL compression
algorithm
• Triggers an Application code change

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Application Impact (Example : JAVA)
getBytes() BYTEA
Migration Impact
• Only getBytes() function can be used against BYTEA data type in
PostgreSQL
• getBytes() attempts to read the whole BYTEA into the memory at
a time which will impose performance problems resulting in
excessive usage of Memory and Network bandwidth
• Cannot stream BYTEA in small chunks
BYTEA
Column
Size
Text File 10K
Text File 10K
Image 20M
Image 300M
Text file 100K
Data Migration
getBLOB() BLOB

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Data Migration
Oracle
BLOB
PostgreSQL
LOManual
Migration Impact
• Custom ETLs must be built
• Migration effort and time can be a challenge
• Application code change is expected
• Application functionality and Performance testing

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Application Impact (Example : JAVA)
getBlob()
LO
Migration Impact
• getBlob() function is used against LO similar to Oracle
• getBlob() is the recommended approach for accessing LO (Large Objects)
datatypes from PostgreSQL
• Large Objects can be streamed
• BLOBs can be streamed in multiple chunks resulting in effective performance
Data Migration
BLOB

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Data Migration
Oracle
BLOB with JSON data
PostgreSQL
JSONBManual
Migration Impact
• Can result in an heavy Application design and code change
• Migration effort and time can be a challenge
• Heavy development can be on the cards
SELECT UTL_RAW.CAST_TO_VARCHAR2 select col->>‘value' FROM table;

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Data Migration
Oracle
CLOB with JSON data
PostgreSQL
TEXTAUTOMATIC (Ora2pg)
Migration Impact
• Can result in an heavy Application design and code change
• Migration effort and time can be a challenge
PostgreSQL
JSONB
Manual

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PL/SQL Migration

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PL/SQL Migration
PL/SQL Object PostgreSQL Alternative
Procedures Functions with RETURNS VOID
Functions Functions
Triggers Triggers
Packages Schemas + Temp tables + Functions

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System Packages
SYSTEM PACKAGES PostgreSQL Alternative
DBMS* PACKAGES • ORAFCE external contrib
module
• PostgreSQL alternatives
• Custom functions

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System Packages
Some of the system packages supported by PostgreSQL
PostgreSQL - ORAFCE
• DBMS_OUTPUT
• DBMS_ALERT
• DBMS_PIPE
• DBMS_UTILITY
• DBMS_ASSERT
• DBMS_RANDOM

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System Packages
Challenges in migrating DBMS_JOBS system package
Oracle PostgreSQL
• DBMS_JOBS • pg_agent

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Packages
Packages
PACKAGES PostgreSQL Alternative
PACKAGE DEFINITION SCHEMA
GLOBAL VARIABLES TEMP TABLES
PACKAGE BODY FUNCTION(S)

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Migrating Oracle Queries

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Oracle Hints
Oracle Hints
PostgreSQL
NO HINTS
Migration Impact
• HINTS are not supported in PostgreSQL - NOT REQUIRED 
• Code changes required
• Functionality testing
• Performance testing and Benchmarking required

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Oracle Outer joins
Oracle LEFT OUTER JOIN PostgreSQL LEFT OUTER JOIN
SELECT e.ename, d.dname 2 FROM emp e,
dept d 3 WHERE e.deptno = d.deptno(+);
SELECT e.ename, d.dname FROM emp e
LEFT OUTER JOIN
dept d ON (e.deptno = d.deptno);
Migration Impact
• Application code change
• Functionality testing

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Oracle PostgreSQL
SELECT sequencename.nextval from dual SELECT nextval(‘sequencename');
SELECT sequencename.currval from dual SELECT currval(‘sequencename’);
Sequences

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Subqueries
Every subquery has to have an alias
Oracle PostgreSQL
select * from (select * from emp) select * from (select * from emp) as foo

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Implicit Type Casting
 Implicit type casting can be highly beneficial and can help reduced the
need to change the application code to a greater extent
 Increases the possibility usage of Indexes
 Avoids the need to create function-based Indexes
Table PostgreSQL
select * from table where col1::int > 2;
create cast(varchar as integer) with inout as implicit;
select * from table where col1 > 2;
Oracle
select * from table where col1 > 2;
varchar varchar
2 Sydney
3 London
4 Singapore
3 London
4 Singapore
3 London
4 Singapore

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Hierarchical Queries
Oracle
Hierarchical Queries
PostgreSQL
Hierarchical Queries
(There is no straight forward way)
Oracle PostgreSQL
• CONNECT BY, START WITH,
SYS_CONNECT_BY_PATH and
CONNECT_BY_ROOT
• WITH RECURSIVE
• connectby() function part of tablefunc
contrib module
• UNION and UNION ALL
• pl-pgsql functions

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Application Migration Challenges

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JDBC Driver
Oracle JDBC Driver
PostgreSQL
JDBC Driver
Migration Impact
AUTOCOMMIT OFF
• Application behaviour is different in autocommit off mode
• Multiple transactions will be automatically part of a transaction block, which means COMMIT ALL or
NONE
• Heavy application code change may be required
• Functionality testing
• Performance testing
• Changing legacy code might impose further more challenges

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Monitoring
Automation
Integration
Database Environment

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PostgreSQL Capabilities

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Replication Slots
wal_keep_segments
Earlier to 9.4
upgrade
Replication Slots
9.4

45. WAL Optimization
 PostgreSQL – 9.4, the amount of WAL data to be written has been reduced in-case of
updates. This results in IO performance gain and reduction in the size of WAL files.
 Example : When one column part of a 4 column row gets updated, only the updated
part of the row is written to WAL files
Table WAL File (9.3)
Row 1
Row 2
Col 1 Col 2 Col 3 Col 4
Row 1
Row 2
WAL File (9.4 )
Row 1
Row 2
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WAL Optimization
 Table Size – 19GB
 4 Columns
 Updated 1 column
0
5
10
15
20
25
WALs (GB) in 9.3 WALs (GB) in 9.4
Series1

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Questions ?