Oracle Support

LiveCompare can be used to compare data from an Oracle database against any number of PostgreSQL or BDR databases.

For example, you can define technology = oracle in a data connection. Other settings can then be used to define the connection to Oracle:

  • host

  • port

  • service

  • user

  • password

All other data connections are required to be PostgreSQL.

Here is a simple example of comparison between an Oracle database versus a PostgreSQL database:

[General Settings]
logical_replication_mode = off
full_comparison_mode = on
max_parallel_workers = 4
oracle_user_tables_only = on
oracle_ignore_unsortable = on
column_intersection = on
force_collate = C
difference_tie_breakers = oracle

[Oracle Connection]
technology = oracle
host = 127.0.0.1
port = 1521
service = XE
user = LIVE
password = live

[Postgres Connection]
technology = postgresql
dsn = dbname=liveoracle user=william

[Output Connection]
dsn = dbname=liveoutput user=william

[Table Filter]
schemas = schema_name = 'live'

Here the schema_name in Oracle is the user table sandbox. All table names are schema-qualified by default:

  • Postgres: <schema_name> . <table_name>

  • Oracle: <user> . <table_name

It is possible to disable schema-qualified table names by setting schema_qualified_table_names = off. This can be done only if oracle_user_tables_only = on, which means that LiveCompare will search only on tables that belong to the Oracle user that is connected. When schema-qualified table names is disabled, then on Postgres you need to have set a default search_path on your role or configuration, or you can use the connection start_query parameter to set an appropriate search_path, for example:

[General Settings]
logical_replication_mode = off
full_comparison_mode = on
max_parallel_workers = 4
oracle_user_tables_only = on
oracle_ignore_unsortable = on
column_intersection = on
force_collate = C
difference_tie_breakers = oracle
schema_qualified_table_names = off

[Oracle Connection]
technology = oracle
host = 127.0.0.1
port = 1521
service = XE
user = LIVE
password = live

[Postgres Connection]
technology = postgresql
dsn = dbname=liveoracle user=william
start_query = SET search_path = myschema1, myschema2, public

[Output Connection]
dsn = dbname=liveoutput user=william

[Table Filter]
tables = table_name in ('mytable1', 'mytable2')

When schema_qualified_table_names = off, then you can use non-qualified table names in the Table Filter, Row Filter and Column Filter too.

Also please note that the Output Connection is required to write progress and reporting information from LiveCompare.

If you need to compare a BDR database against Oracle, and you want to take advantage about Initial Connection, node_name and replication_sets features (as explained earlier), then you can point the last data connection to Oracle, like this:

[General Settings]
logical_replication_mode = bdr
full_comparison_mode = on
max_parallel_workers = 4
oracle_user_tables_only = on
oracle_ignore_unsortable = on
column_intersection = on
force_collate = C
difference_tie_breakers = oracle

[Initial Connection]
dsn = port=5432 dbname=live user=postgres

[BDR Connection]
node_name = node1

[Oracle Connection]
technology = oracle
host = 127.0.0.1
port = 1521
service = XE
user = LIVE
password = live

[Output Connection]
dsn = port=5432 dbname=liveoutput user=postgres

[Table Filter]
replication_sets = set_name = 'bdrgroup'

It is also possible to compare a whole BDR cluster against a single Oracle database, for example:

[General Settings]
logical_replication_mode = bdr
full_comparison_mode = on
max_parallel_workers = 4
oracle_user_tables_only = on
oracle_ignore_unsortable = on
column_intersection = on
force_collate = C
difference_tie_breakers = oracle
all_bdr_nodes = on

[Initial Connection]
dsn = port=5432 dbname=live user=postgres

[Oracle Connection]
technology = oracle
host = 127.0.0.1
port = 1521
service = XE
user = LIVE
password = live

[Output Connection]
dsn = port=5432 dbname=liveoutput user=postgres

[Table Filter]
replication_sets = set_name = 'bdrgroup'

Requirements

Please note that LiveCompare works on PostgreSQL databases out-of-the-box, without needing to install any additional software.

But in order to be able to connect to Oracle, LiveCompare requires additional software:

Oracle Instant Client

You need to download and install Oracle Instant Client (or extract it to a specific folder, depending on the operating system you use):

  • MacOSX: Download Oracle Instant Client (64-bit) and extract in ~/lib;

  • Linux: Download Oracle Instant Client (32-bit) (64-bit) and install it on your system, then set LD_LIBRARY_PATH;

  • Windows: Download Oracle Instant Client (32-bit) (64-bit) and extract it into the LiveCompare folder.

cx_Oracle Python module

The Python module cx_Oracle is required to be installed and available on your system so that LiveCompare is able to connect to an Oracle Database.

Currently cx_Oracle is not installable from Linux distribution repositories, so please follow the instructions in cx_Oracle website to install it on your system.

As LiveCompare is recommended to be executed under the postgres operating system user, then it is possible to install the cx_Oracle module through PIP only for the postgres user, with the following command:

pip3 install --user cx_Oracle --upgrade

Differences

If LiveCompare finds any difference, it will generate a DML script only to be applied on the PostgreSQL connections. A DML script to be applied in the Oracle connection is not generated.

BLOB and CLOB Data Types

LiveCompare is able to compare CLOB fields from Oracle, provided that the equivalent field in PostgreSQL is of type text. The same goes for BLOB fields from Oracle, the equivalent in PostgreSQL should be of type bytea.

However, by default LiveCompare does not handle BLOB/CLOB fields if they are in the primary key, or if the table has no primary key. If that’s the case, then the table will be ignored, and in LiveCompare logs you will see a message like this:

ORA-00932: inconsistent datatypes: expected - got BLOB

It is possible to workaround this behaviour by telling LiveCompare to ignore BLOB/CLOB fields if table has no primary key, by enabling these 2 settings in the General Settings section:

oracle_ignore_unsortable = on
column_intersection = on

Incompatible Collation

On Oracle, generally we have the following initialization parameters set:

NLS_COMP = BINARY
NLS_SORT = BINARY

This means that, regardless of the NLS_LANG and other language settings, all ORDER BY operations in Oracle are performed using the character binary code.

In Postgres, the equivalent collation that shows the same behavior is the C collation. If your Postgres database was initialized in a different collation, then by default LiveCompare might find issues when sorting PK values, which can lead to false positives.

To workaround that, it is possible to force a collation (say, the C collation) in Postgres, so it matches the same sort behavior from Oracle:

force_collate = C

If LiveCompare detects that the comparison session involves Oracle and PostgreSQL, then LiveCompare already sets force_collate = C, unless the user has set it to any other value.

Common Hash

By default, LiveCompare has comparison_algorithm = block_hash, even when comparing PostgreSQL versus Oracle. However, a “common hash” is built following these rules:

  • The row is fully represented as text, by concatenating all column values;

  • On Postgres side, timestamp, numeric and bytea data types are handled to mimic Oracle;

  • This way, the full row representation is then hashed using MD5 on both sides;

  • This allows using comparison_algorithm set to block_hash and row_hash;

  • If there are any mismatches when using block_hash, LiveCompare will automatically fall back to row_hash and then full_row, as it would on a Postgres versus Postgres comparison.

  • The BLOB, CLOB and NCLOB fields on Oracle are limited to the first 2000 characters only (comparison_algorithm = full_row allows comparison of the entire BLOB and CLOB);

  • On Oracle, the full row representation should not be wider than 4000 characters.

If the full row representation is wider than 4000 characters, LiveCompare will abort the comparison for that specific table, and you will see the following error message in the logs:

ORA-01489: result of string concatenation is too long

Further LiveCompare versions will fall back to full_row comparison on these specific tables. For now, a workaround would be to configure a separate comparison sessions on these tables only, using comparison_algorithm = full_row.

Also note that the Common Hash requires the standard_hash function on Oracle, which is available only on Oracle 12c and newer. On Oracle 10g and 11c, please use comparison_algorithm = full_row.