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Immutable Table Enhancements in Oracle Database 23c

Immutable tables were introduced to Oracle 21.3 and 19.11 at the same time, so it could be considered a 19c and 21c new feature. This article demonstrates the enhancements to immutable tables in Oracle 23c. There are some fundamental changes to immutable tables in Oracle 23c, so some of the previous article will be replicated here.

An immutable table is a tamper-proof, insert-only table with an associated table-level and row-level retention period. They are similar to blockchain tables, but the rows are not chained using cryptographic hashes.

When learning about immutable tables, be careful not to set excessively long retention periods, or you will have to wait a long time to drop your test tables.

Create an Immutable Table
Alter an Immutable Table
Blocked DML and DDL Operations
Add/Drop Columns
Control Long Idle Retention Times
DBMS_IMMUTABLE_TABLE Package
Considerations

Create an Immutable Table

In addition to adding the IMMUTABLE keyword to the CREATE TABLE command, there are two immutable clauses.

The NO DROP clause determines how long the table is protected from being dropped. If the table has no rows it can still be dropped. Unlike the initial releases of blockchain tables, the NO DROP clause also prevents the table being dropped via a DROP USER ... CASCADE command.

NO DROP [ UNTIL number DAYS IDLE ]

NO DROP : The table can't be dropped. Be careful about using this setting during testing.
NO DROP UNTIL number DAYS IDLE : The table can't dropped until there have been no new rows inserted for the specified number of days. You may prefer to use 0 or 1 as the number of days during testing this functionality.

The NO DELETE clause determines the retention period. How long each row will be protected from deletion.

NO DELETE { [ LOCKED ] | (UNTIL number DAYS AFTER INSERT [ LOCKED ]) }

NO DELETE : Each row is retained forever. The absence of the LOCKED keyword implies the setting can be changed with the ALTER TABLE command, but it can't. Retention periods can only be increased.
NO DELETE LOCKED : Same as NO DELETE.
NO DELETE UNTIL number DAYS AFTER INSERT : Each row is protected from deletion for the specified number of days, but this setting can be increased using the ALTER TABLE command. Minimum 16 days.
NO DELETE UNTIL number DAYS AFTER INSERT LOCKED : Each row is protected from deletion for the specified number of days, and this setting can't be changed using the ALTER TABLE command. Minimum 16 days.

In Oracle 23c we have the option of using the original "V1" version, which is the default, or the new "V2" version, which supports additional functionality. The following example creates two tables. One of each version.

drop table if exists it_t1 purge;

create immutable table it_t1 (
  id            number,
  fruit         varchar2(20),
  quantity      number,
  created_date  date,
  constraint it_t1_pk primary key (id)
)
no drop until 0 days idle
no delete until 16 days after insert;


drop table if exists it_t2 purge;

create immutable table it_t2 (
  id            number,
  fruit         varchar2(20),
  quantity      number,
  created_date  date,
  constraint it_t2_pk primary key (id)
)
no drop until 0 days idle
no delete until 16 days after insert version "v2";

Checking the USER_TAB_COLS view shows us several invisible columns have been added to our column list. The hidden columns are the same as those of a blockchain table, but unlike blockchain tables, only the ORABCTAB_CREATION_TIME$ and ORABCTAB_USER_NUMBER$ columns are populated. The rest of the columns are set to null. The hidden columns are described here. Notice the "V2" table has twice the number of hidden columns compared to the "V1" table.

set linesize 120 pagesize 50
column column_name format a45
column data_type format a27
column hidden_column format a13

select internal_column_id,
       column_name,
       data_type,
       data_length,
       hidden_column
FROM   user_tab_cols       
WHERE  table_name = 'IT_T1'
ORDER BY internal_column_id;

INTERNAL_COLUMN_ID COLUMN_NAME                                   DATA_TYPE                   DATA_LENGTH HIDDEN_COLUMN
------------------ --------------------------------------------- --------------------------- ----------- -------------
                 1 ID                                            NUMBER                               22 NO
                 2 FRUIT                                         VARCHAR2                             20 NO
                 3 QUANTITY                                      NUMBER                               22 NO
                 4 CREATED_DATE                                  DATE                                  7 NO
                 5 ORABCTAB_INST_ID$                             NUMBER                               22 YES
                 6 ORABCTAB_CHAIN_ID$                            NUMBER                               22 YES
                 7 ORABCTAB_SEQ_NUM$                             NUMBER                               22 YES
                 8 ORABCTAB_CREATION_TIME$                       TIMESTAMP(6) WITH TIME ZONE          13 YES
                 9 ORABCTAB_USER_NUMBER$                         NUMBER                               22 YES
                10 ORABCTAB_HASH$                                RAW                                2000 YES
                11 ORABCTAB_SIGNATURE$                           RAW                                2000 YES
                12 ORABCTAB_SIGNATURE_ALG$                       NUMBER                               22 YES
                13 ORABCTAB_SIGNATURE_CERT$                      RAW                                  16 YES
                14 ORABCTAB_SPARE$                               RAW                                2000 YES

14 rows selected.

SQL>


select internal_column_id,
       column_name,
       data_type,
       data_length,
       hidden_column
FROM   user_tab_cols       
WHERE  table_name = 'IT_T2'
ORDER BY internal_column_id;

INTERNAL_COLUMN_ID COLUMN_NAME                                   DATA_TYPE                   DATA_LENGTH HIDDEN_COLUMN
------------------ --------------------------------------------- --------------------------- ----------- -------------
                 1 ID                                            NUMBER                               22 NO
                 2 FRUIT                                         VARCHAR2                             20 NO
                 3 QUANTITY                                      NUMBER                               22 NO
                 4 CREATED_DATE                                  DATE                                  7 NO
                 5 ORABCTAB_INST_ID$                             NUMBER                               22 YES
                 6 ORABCTAB_CHAIN_ID$                            NUMBER                               22 YES
                 7 ORABCTAB_SEQ_NUM$                             NUMBER                               22 YES
                 8 ORABCTAB_CREATION_TIME$                       TIMESTAMP(6) WITH TIME ZONE          13 YES
                 9 ORABCTAB_USER_NUMBER$                         NUMBER                               22 YES
                10 ORABCTAB_HASH$                                RAW                                2000 YES
                11 ORABCTAB_SIGNATURE$                           RAW                                2000 YES
                12 ORABCTAB_SIGNATURE_ALG$                       NUMBER                               22 YES
                13 ORABCTAB_SIGNATURE_CERT$                      RAW                                1000 YES
                14 ORABCTAB_SPARE$                               RAW                                2000 YES
                15 ORABCTAB_PDB_GUID$                            RAW                                2000 YES
                16 ORABCTAB_ROW_VERSION$                         NUMBER                               22 YES
                17 ORABCTAB_LAST_ROW_VERSION_NUMBER$             RAW                                   1 YES
                18 ORABCTAB_USER_CHAIN_HASH$                     RAW                                2000 YES
                19 ORABCTAB_DELEGATE_SIGNATURE$                  RAW                                2000 YES
                20 ORABCTAB_DELEGATE_SIGNATURE_ALG$              NUMBER                               22 YES
                21 ORABCTAB_DELEGATE_SIGNATURE_CERT$             RAW                                1000 YES
                22 ORABCTAB_DELEGATE_USER_NUMBER$                NUMBER                               22 YES
                23 ORABCTAB_COUNTERSIGNATURE$                    RAW                                2000 YES
                24 ORABCTAB_COUNTERSIGNATURE_ALG$                NUMBER                               22 YES
                25 ORABCTAB_COUNTERSIGNATURE_CERT$               RAW                                1000 YES
                26 ORABCTAB_COUNTERSIGNATURE_ROW_FORMAT_VERSION$ VARCHAR2                           4000 YES
                27 ORABCTAB_COUNTERSIGNATURE_ROW_FORMAT_FLAG$    NUMBER                               22 YES
                28 ORABCTAB_TS$                                  TIMESTAMP(6)                         13 YES

28 rows selected.

SQL>

The {CDB|DBA|ALL|USER}_IMMUTABLE_TABLES views display information about immutable tables. It's a view over the SYS.IMMUTABLE_TABLE$ table.

column table_name format a10
column row_retention format 9999999999999
column row_retention_locked format a20
column table_inactivity_retention format 99999999999999999999999999

SELECT table_name,
       row_retention,
       row_retention_locked, 
       table_inactivity_retention
FROM   user_immutable_tables
order by 1;

TABLE_NAME  ROW_RETENTION ROW_RETENTION_LOCKED  TABLE_INACTIVITY_RETENTION
---------- -------------- -------------------- ---------------------------
IT_T1                  16 NO                                             0
IT_T2                  16 NO                                             0

SQL>

Alter an Immutable Table

The following operations perform the same on "V1" and "V2" blockchain tables.

The NO DROP clause can be altered using the ALTER TABLE command, as long as the retention period is not reduced. Be careful not to set it too large when testing.

alter table it_t1 no drop until 1 days idle;

Regardless of the current drop delay setting, an attempt to switch to the maximum value of NO DROP causes an error. This is because of the BLOCKCHAIN_TABLE_RETENTION_THRESHOLD parameter discussed below.

alter table it_t1 no drop;
*
ERROR at line 1:
ORA-05807: Blockchain or immutable table "TESTUSER1"."IT_T1" cannot have idle retention greater than 16 days.

SQL>

Assuming it was not defined as locked, the NO DELETE clause can be modified using the ALTER TABLE command, as long as the retention period is not reduced. We currently have a row retention period of 16 days. In the example below we increase that value to 32. When we subsequently attempt to lower the value to 16 it gives an error.

-- Increase to 32 days.
alter table it_t1 no delete until 32 days after insert;

Table IT_T1 altered.

SQL>


-- Decrease to 16 days (fail).
alter table it_t1 no delete until 16 days after insert;

Error report -
ORA-05732: retention value cannot be lowered

SQL>

We can set the row retention to NO DELETE, which means rows will be kept forever. Remember, we can't reduce this value once is it set, so take care using such a long retention period.

alter table it_t1 no delete;

We'll drop and recreate the "V1" immutable table. We've not inserted any rows into it yet, so despite the retention times the following command works.

drop table if exists it_t1 purge;

create immutable table it_t1 (
  id            number,
  fruit         varchar2(20),
  quantity      number,
  created_date  date,
  constraint it_t1_pk primary key (id)
)
no drop until 0 days idle
no delete until 16 days after insert;

Blocked DML and DDL Operations

As you would expect for an insert-only table, all DML and DDL operations that would result in row data being amended or deleted are prevented for an immutable table.

The following example shows a successful insert, then some unsuccessful DML statements.

-- INSERT
insert into it_t2 (id, fruit, quantity, created_date ) values (1, 'apple', 20, sysdate);

1 row inserted.

SQL> commit;

Commit complete.

SQL>


-- UPDATE
update it_t2 set quantity = 10 where id = 1;
       *
ERROR at line 1:
ORA-05715: operation not allowed on the blockchain or immutable table

SQL>


-- DELETE
delete from it_t2 where id = 1;
            *
ERROR at line 1:
ORA-05715: operation not allowed on the blockchain or immutable table

SQL>

Some DDL statements that could alter the contents of the data are also prevented. Here is an example of the TRUNCATE statement.

truncate table it_t1;
               *
ERROR at line 1:
ORA-05715: operation not allowed on the blockchain or immutable table

SQL>

Extending existing columns is fine.

alter table it_t2 modify (fruit varchar2(25));

Table altered.

SQL>

Add/Drop Columns

In a "V1" immutable table adding new columns or dropping existing columns is not allowed.

-- Add column
alter table it_t1 add (additional_info varchar2(50));
*
ERROR at line 1:
ORA-05715: operation not allowed on the blockchain or immutable table

SQL>


-- Drop column.
alter table it_t1 drop column quantity;
*
ERROR at line 1:
ORA-05715: operation not allowed on the blockchain or immutable table

SQL>

In a "V2" immutable table we can add new columns and drop existing columns. The dropped columns are marked as hidden, rather than actually being dropped.

-- Add column
alter table it_t2 add (additional_info varchar2(50));

Table altered.

SQL>


-- Drop column.
alter table it_t2 drop column additional_info;

Table altered.

SQL>

Control Long Idle Retention Times

In Oracle 23c the maximum idle retention time for the table is controlled by the BLOCKCHAIN_TABLE_RETENTION_THRESHOLD parameter.

show parameter blockchain_table_retention_threshold

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
blockchain_table_retention_threshold integer     16
SQL>

This means we can't set the NO DROP clause to a value longer than 16 days.

drop table if exists it_t3 purge;

create immutable table it_t3 (
  id            number,
  fruit         varchar2(20),
  quantity      number,
  created_date  date,
  constraint it_t3_pk primary key (id)
)
no drop until 32 days idle
no delete until 16 days after insert;
*
ERROR at line 1:
ORA-05807: Blockchain or immutable table "TESTUSER1"."IT_T3" cannot have idle retention greater than 16 days.

SQL>

We can either increase this setting, or grant the user the TABLE RETENTION privilege, which allows the user to ignore this limit.

conn sys/SysPassword1@//localhost:1521/freepdb1 as sysdba

grant table retention to testuser1;

conn testuser1/testuser1@//localhost:1521/freepdb1

Now we can ignore the limit.

drop table if exists itt_t3 purge;

create immutable table it_t3 (
  id            number,
  fruit         varchar2(20),
  quantity      number,
  created_date  date,
  constraint it_t3_pk primary key (id)
)
no drop until 32 days idle
no delete until 16 days after insert;

Table created.

SQL>

DBMS_IMMUTABLE_TABLE Package

The DBMS_IMMUTABLE_TABLE package is used for maintenance of immutable tables.

The DELETE_EXPIRED_ROWS procedure removes any rows that are beyond the retention period. They can't be removed using a normal DELETE statement.

set serveroutput on
declare
  l_rows  number;
begin
  dbms_immutable_table.delete_expired_rows(
    schema_name            => 'testuser1',
    table_name             => 'it_t1',
    before_timestamp       => null,
    number_of_rows_deleted => l_rows);

  dbms_output.put_line('Rows Deleted=' || l_rows);
end;
/
Rows Deleted=0

PL/SQL procedure successfully completed.

SQL>

Alternatively, we can limit the deletion by date. The rows will only be deleted if they are outside the retention period, and match the date criteria.

set serveroutput on
declare
  l_rows  number;
begin
  dbms_immutable_table.delete_expired_rows(
    schema_name            => 'testuser1',
    table_name             => 'it_t1',
    before_timestamp       => systimestamp - 60,
    number_of_rows_deleted => l_rows);

  dbms_output.put_line('Rows Deleted=' || l_rows);
end;
/
Rows Deleted=0

PL/SQL procedure successfully completed.

SQL>

Considerations

There are a number of things to consider when using immutable tables.

Immutable tables can be indexed and partitioned in the normal manner.
There are a number of general restrictions associated with blockchain tables, described here.

I guess the main question should be, why would you use an immutable table?

If you need an insert-only tamper proof table in your application generally, this could be the solution.
If you want the added security of cryptographic hashes, you might want to consider blockchain tables.

For more information see:

Hope this helps. Regards Tim...

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