How Do Global and Private Temporary Tables Differ in Oracle Database Management?

Oracle Temporary Tables: A closer look at transient data storage

In the field of relational database management, creating temporary tables is a common requirement in various data processing scenarios. In Microsoft SQL Server, the syntax for creating temporary tables using the DECLARE statement is relatively simple. However, in Oracle database, the approach is different and needs further exploration.

Oracle provides two types of temporary tables: global temporary tables and private temporary tables. Global temporary tables are represented by the GLOBAL TEMP keyword in their creation statements and are visible and accessible across multiple user sessions. On the other hand, private temporary tables introduced in Oracle 18c are exclusive to the creation session and are often used for performance optimization.

Create global temporary table

To create a global temporary table in Oracle, use the following syntax:

<code class="language-sql">CREATE GLOBAL TEMPORARY TABLE 表名 (列定义)
ON COMMIT [DELETE ROWS | PRESERVE ROWS]</code>

By default, data in global temporary tables is deleted when the transaction commits. However, the ON COMMIT clause allows you to specify the desired behavior:

• DELETE ROWS: Delete data when submitting.
• PRESERVE ROWS: Data remains even after submission, allowing other sessions to access it.

Consider the following example:

<code class="language-sql">CREATE GLOBAL TEMPORARY TABLE sales_temp (
  order_id NUMBER,
  quantity NUMBER
)
ON COMMIT PRESERVE ROWS;</code>

Create private temporary table

Private temporary tables, available in Oracle 18c and later, provide enhanced performance by storing data in memory within the creation session. They are created using the following syntax:

<code class="language-sql">CREATE PRIVATE TEMPORARY TABLE 表名 (列定义)</code>

Here is an example:

<code class="language-sql">CREATE PRIVATE TEMPORARY TABLE order_temp AS
  SELECT order_id, product_id, quantity
  FROM orders
  WHERE order_date = '2023-08-22';</code>

Notes and Alternatives

While temporary tables can be valuable in certain situations, they must be used with caution. Frequent creation and deletion of temporary tables can cause performance overhead to the database.

An alternative to temporary tables is to use inline views (CTE), which allow you to create a temporary result set in one SQL statement. This approach can potentially improve performance and reduce the need for separate temporary table creation.

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