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.

If you've come to Oracle from a different database engine, like SQL Server, you might be confused by Oracle's idea of temporary tables. In SQL Server developers will regularly create a temporary table to do some work and drop it. 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.

- Global Temporary Tables : Available since Oracle 8i. Discussed here.
- Private Temporary Tables : Available since Oracle 18c and subject of this article.

◉ 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 my_temp_table (
  id           NUMBER,
  description  VARCHAR2(20)
);

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

SQL>

◉ Creation of Private Temporary Tables

The syntax for creating a private temporary table will look familiar if you have used 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_my_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_my_temp_table VALUES (1, 'ONE');

SELECT COUNT(*) FROM ora$ptt_my_temp_table;

  COUNT(*)
----------
         1

SQL>

-- Commit and check contents.
COMMIT;

SELECT COUNT(*) FROM ora$ptt_my_temp_table;
SELECT COUNT(*) FROM ora$ptt_my_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_my_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_my_temp_table VALUES (1, 'ONE');

SELECT COUNT(*) FROM ora$ptt_my_temp_table;

  COUNT(*)
----------
         1

SQL>


-- Commit and check contents.
COMMIT;

SELECT COUNT(*) FROM ora$ptt_my_temp_table;

  COUNT(*)
----------
         1

SQL>


-- Reconnect and check contents of GTT.
CONN test/test@pdb1

SELECT COUNT(*) FROM ora$ptt_my_temp_table;
SELECT COUNT(*) FROM ora$ptt_my_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.

We can also create private temporary tables using the CTAS method.

CREATE PRIVATE TEMPORARY TABLE ora$ptt_emp AS
SELECT * FROM emp;

◉ 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 silly 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_my_temp_table (
              id           NUMBER,
              description  VARCHAR2(20)
            )
            ON COMMIT DROP DEFINITION';

  EXECUTE IMMEDIATE l_sql;

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

  EXECUTE IMMEDIATE 'SELECT description INTO :l_return FROM ora$ptt_my_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>

◉ Views

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.
- Primary keys, or any constraint that requires an index, are not allows on private temporary tables.
- Columns can't have default values.
- Private temporary tables can't be accessed via database links.