Private Temporary Tables in Oracle Database 18c

Oracle 18c introduced the concept of a private temporary table, a memory-based temporary table that is dropped at the end of the session or transaction depending on the setup.

In Oracle a Global Temporary Table (GTT) is a permanent metadata object that holds rows in temporary segments on a transaction-specfic or session-specific basis. It is not considered normal to create and drop GTTs on the fly. With the introduction of private temporary tables, Oracle has an option similar to that seen in other engines, where the table object itself is temporary, not just the data.

Temporary Tables

Oracle support two types of temporary tables.

1. Global Temporary Tables : Available since Oracle 8i. details here.
2. Private Temporary Tables : Available since Oracle 18c.

Naming Private Temporary Tables

The PRIVATE_TEMP_TABLE_PREFIX initialisation parameter, which defaults to "ORA$PTT_", defines the prefix that must be used in the name when creating the private temporary table. In the following example we create a private temporary table without using the correct prefix in the name, which results in an error.

CREATE PRIVATE TEMPORARY TABLE ptt_temp_table (
  id          NUMBER,
  description VARCHAR2(20)
);

CREATE PRIVATE TEMPORARY TABLE ptt_temp_table (
                               *
ERROR at line 1:
ORA-00903: invalid table name

SQL>

Creation of Private Temporary Tables

The syntax for creating a private temporary is similar to the global temporary tables.

The ON COMMIT DROP DEFINITION clause, the default, indicates the table should be dropped at the end of the transaction, or the end of the session.

CREATE PRIVATE TEMPORARY TABLE ora$ptt_temp_table (
  id           NUMBER,
  description  VARCHAR2(20)
)
ON COMMIT DROP DEFINITION;


-- Insert, but don't commit, then check contents of PTT.
INSERT INTO ora$ptt_temp_table VALUES (1, 'ONE');

SELECT COUNT(*) FROM ora$ptt_temp_table;

  COUNT(*)
----------
         1

SQL>

-- Commit and check contents.
COMMIT;

SELECT COUNT(*) FROM ora$ptt_temp_table;
SELECT COUNT(*) FROM ora$ptt_temp_table;
                     *
ERROR at line 1:
ORA-00942: table or view does not exist

SQL>

In contrast, the ON COMMIT PRESERVE DEFINITION clause indicates the table and any data should persist beyond the end of the transaction. The table will be dropped at the end of the session.

CREATE PRIVATE TEMPORARY TABLE ora$ptt_temp_table (
  id           NUMBER,
  description  VARCHAR2(20)
)
ON COMMIT PRESERVE DEFINITION;


-- Insert, but don't commit, then check contents of PTT.
INSERT INTO ora$ptt_temp_table VALUES (1, 'ONE');

SELECT COUNT(*) FROM ora$ptt_temp_table;

  COUNT(*)
----------
         1

SQL>


-- Commit and check contents.
COMMIT;

SELECT COUNT(*) FROM ora$ptt_temp_table;

  COUNT(*)
----------
         1

SQL>


-- Reconnect and check contents of PTT.
CONN scott/tiger@oradb

SELECT COUNT(*) FROM ora$ptt_temp_table;
SELECT COUNT(*) FROM ora$ptt_temp_table
                     *
ERROR at line 1:
ORA-00942: table or view does not exist

SQL>

The above output shows the table persists beyond the commit, but is dropped when we disconnect and create a new session.

Private Temporary Tables and PL/SQL

It doesn't make sense for a permanent PL/SQL object to directly reference a temporary object, as it would not exist at compile time. If you want to use a private temporary table from a permanent object it would have to be done using dynamic SQL. The following example creates a stored function which uses a private temporary table.

CREATE OR REPLACE FUNCTION ptt_test (p_id IN NUMBER)
RETURN VARCHAR2
AS
  l_sql     VARCHAR2(32767);
  l_return  VARCHAR2(30);
BEGIN
  l_sql := 'CREATE PRIVATE TEMPORARY TABLE ora$ptt_temp_table (
              id           NUMBER,
              description  VARCHAR2(20)
            )
            ON COMMIT DROP DEFINITION';

  EXECUTE IMMEDIATE l_sql;

  EXECUTE IMMEDIATE q'[INSERT INTO ora$ptt_temp_table VALUES (1, 'ONE')]';

  EXECUTE IMMEDIATE 'SELECT description INTO :l_return FROM ora$ptt_temp_table WHERE id = :id' INTO l_return USING p_id;

  RETURN l_return;
END;
/

The function works as expected.

SET SERVEROUTPUT ON
BEGIN
  DBMS_OUTPUT.put_line('ptt_test(1) = ' || ptt_test(1));
END;
/
ptt_test(1) = ONE

PL/SQL procedure successfully completed.

SQL>

Data Dictionary

Private temporary tables are memory-based, so there is no metadata recorded in the data dictionary. As a result you can't use the USER_TABLES view to display the list of private temporary tables in the current session. The following views are available to display information about private temporary tables.

• DBA_PRIVATE_TEMP_TABLES : All private temporary tables in the database.
• USER_PRIVATE_TEMP_TABLES : Private temporary tables in the current session.

Restrictions

Private temporary tables share the limitations of global temporary tables (see here), but there are also additional restrictions.

• The table name must begin with the prefix defined in the PRIVATE_TEMP_TABLE_PREFIX initialisation parameter. The default is "ORA$PTT_".
• Permanent objects can't reference private temporary tables directly.
• Indexes, materialized views, and zone maps are not allowed on private temporary tables.
• Columns can't have default values.
• Private temporary tables can't be accessed via database links.