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Where Temporary Tables Are Stored In Oracle

Applications often use some form of temporary data store for processes that are to complicated to complete in a single pass. Often, these temporary stores are defined as database tables or PL/SQL tables. From Oracle 8i onward, the maintenance and management of temporary tables can be delegated to the server by using Global Temporary Tables.

where temporary tables are stored in oracle

Although the data in a GTT is written to the temporary tablespace, the associated undo is still written to the normal undo tablespace, which is itself protected by redo, so using a GTT does not reduce undo and the redo associated with protecting the undo tablespace.

If you've read the previous section, you will already know the relationship between global temporary tables and redo. The data in a GTT is written to the temporary tablespace, which is not directly protected by redo, so using a GTT improves performance by reducing redo generation. Unfortunately, prior to Oracle 12c, all undo associated with DML against a GTT is written to the normal undo tablespace, which is itself protected by redo. As a result, using a GTT reduces the amount of redo generation, but does not eliminate it. Another why of describing this is, using a GTT removes direct redo generation, but not indirect redo generation cause by undo.

A new variation of temporary tables has been introduced in Oracle 18c. A private temporary table is a memory-based temporary table that is dropped at the end of the session or transaction depending on the setup. You can read more about them here.

Unlike temporary tables from other database products such as MySQL and SQL Server, global temporary tables in Oracle are permanent database objects that store data on disk and visible to all sessions.

12c note: Starting in 12c, Oracle will allow you to invoke session-level dbms_stats to gather statistics specific to your own global temporary table. Prior to 12c, statistics were shared from a master copy of the CBO statistics. In addition to data dictionary queries, global temporary tables can dramatically improve the performance of certain SQL self-join queries that summarize data values. The Elapsed-Time Section of the Date Range Report is a very sophisticated DBA report, and one that can run for many hours without the use of global temporary tables because of Oracle's use of the CARTESIAN access method. However, with the use of global temporary tables, the table and index counts can be summarized and saved in the temp tables for fast analysis. We also use the same technique to sum the number of bytes in all tables and indexes into temporary tables, and then quickly interrogate the summary tables for total sizes of our database. You can use the global temporary tables (GTT) syntax to improve the speed of queries that perform complex summarization activities.

select table_name from all_tables where temporary = 'Y'; Upon close examination, we see that we create global temporary tables to hold the total counts, and we also create two temporary tables to hold the sum of bytes for each table and index. Once the sums are pre-calculated, it becomes fast and easy for Oracle SQL to compute the total bytes for the whole database. The Oracle database codified this approach starting and Oracle8i with their global temporary tables construct. Here is an example of using global temporary tables. Note that the SQL data from the global temporary tables are preserved and passed (as if it was a "real" table) to a subsequent SQL query:

These tables do not reside in the system catalogs and are notpersistent. Temporary tables exist only during the connection that declaredthem and cannot be referenced outside of that connection. When the connectioncloses, the rows of the table are deleted, and the in-memory description ofthe temporary table is dropped.

Names the temporarytable. If a schema-Name other than SESSION is specified, an error will occur(SQLSTATE 428EK). If the schema-Name is not specified, SESSION is assigned.Multiple connections can define declared global temporary tables with thesame name because each connection has its own unique table descriptor forit.

Specifiesthe action taken on the global temporary table when a rollback operation isperformed. When a ROLLBACK (or ROLLBACK TO SAVEPOINT) operation is performed,if the table was created in the unit of work (or savepoint), the table willbe dropped. If the table was dropped in the unit of work (or savepoint), thetable will be restored with no rows.

Temporary tables come in different flavours including, amongst others, local temporary tables (starting with #), global temporary tables (starting with ##), persistent temporary tables (prefixed by TempDB..), and table variables.(starting with (@)

Another oddity of the local temporary table (and the local temporary stored procedure) is that it has a different name in the metadata to the one you give it in your routine or batch. If the same routine is executed simultaneously by several processes, the Database Engine needs to be able to distinguish between the identically-named local temporary tables created by the different processes. It does this by adding a numeric string to each local temporary table name left-padded by underscore characters. Although you specify the short name such as #MyTempTable, what is actually stored in TempDB is made up of the table name specified in the CREATE TABLE statement and the suffix. Because of this suffix, local temporary table names must be 116 characters or less.

With Local temporary table (names that begin with #), what goes on under the hood is surprisingly similar to table variables. As with Table Variables, Local Temporary tables are private to the process that created it. They cannot therefore be used in views and you cannot associate triggers with them.

You cannot easily tell which session or procedure has created these tables. This is because, if the same stored procedure is executed simultaneously by several processes, the Database Engine needs to be able to distinguish the same tables created by the different processes. The Database Engine does this by internally appending a left-padded numeric suffix to each local temporary table name. The full name of a temporary table as stored in the sys.objects view in TempDB is made up of the table name specified in the CREATE TABLE statement and the system-generated numeric suffix. To allow for the suffix, the table name specified for a local temporary name must be less than 116 characters.

You can associate rules, defaults, and indexes with temporary tables, but you cannot create views on temporary tables or associate triggers with them. You can use a user-defined datatype when creating a temporary table only if the datatype exists in TempDB

Stored procedures can reference temporary tables that are created during the current session. Within a stored procedure, you cannot create a temporary table, drop it, and then create a new temporary table with the same name.

Unlike Oracle, SQL Server does not store the definition of temporary tables permanently in the database catalog views, and this can cause various scope and visibility issues when you use temporary tables.

You can use local temporary tables to emulate Oracle package variables. But due to visibility limitations (see above), you have to create and initialize a local temporary table in the application (for example, right after connection).

Advantages: Relatively easy to use, no conflicts with other sessions, data are cleaned automatically Disadvantages: Needs creation by each application, not easy to track and manage in case of large number of local temporary tables (a lot of packages with variables).

You can use global temporary tables to emulate Oracle package variables. In contract to local temporary tables, once you create a global temporary table, it becomes visible in any procedures and application.

Oracle temporary table which was introduced in Oracle 8i can be defined as a permanent object where the data is stored in a disk and that particular data is immediately deleted after that particular session or transaction has come to an end and for Oracle private temporary tables are stored in the Oracle database memory and each of these tables are visible only to the session which created that particular table and these tables are generally SQL programs embedded in Oracle sessions.

Temporary tables are of two types: Global Temporary Table and Private Temporary Table introduced in Oracle 18c. When we create any temporary table in the Oracle database it is automatically global. So we need to add Global Keyword.

Temporary Table in Oracle as defined earlier is used to store data for some specific task as the temporary table data is deleted as soon as the transaction or session ends or finishes. Suppose we want to extract some data which is not stored in a specific table and is not present in the database. In that case, we can use a procedure to first extract the data from different tables and insert that data into a temporary table during the start of the session or transaction. The data from a temporary table that is created during the starting of the session or transaction can be used throughout the session or throughout the transaction depending upon the permission provided while creating the table. If we give ON COMMIT DELETE ROWS then the table is transaction-specific but if we give ON COMMIT PRESERVE ROWS then the table is session-specific.

In addition to permanent tables, which is the default table type when creating tables, Snowflake supports defining tables as either temporary ortransient. These types of tables are especially useful for storing data that does not need to be maintained for extended periods of time(i.e. transitory data).

Snowflake supports creating temporary tables for storing non-permanent, transitory data (e.g. ETL data, session-specific data). Temporary tablesonly exist within the session in which they were created and persist only for the remainder of the session. As such, they are not visible to otherusers or sessions. Once the session ends, data stored in the table is purged completely from the system and, therefore, is not recoverable, eitherby the user who created the table or Snowflake.


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