Using the global temporary table emulation

The global temporary table emulation is provided to get benefit of the Oracle® GLOBAL TEMPORARY TABLES, by sharing the same table structure with multiple SQL sessions, reducing the cost of the CREATE TABLE statement execution. However, this emulation does not provide the same level of Informix® compatibility as the default emulation, and must be used carefully.

How does the global emulation work?

Informix CREATE TEMP TABLE and SELECT INTO TEMP statements are automatically converted to Oracle "CREATE GLOBAL TEMPORARY TABLE" statements. The original table name is kept, but it gets by default a "TEMPTABS" schema prefix, to share the underlying table structure with other database users.
In order to control the schema where the Oracle global temporary tables are created, use the following FGLPROFILE parameters:
```
dbi.database.dsname.ora.temptables.schema.source = { "login" | "command" }
dbi.database.dsname.ora.temptables.schema.command = "SELECT ..."
```
Note: The purpose of global temporary tables is to create the table once in Oracle and have all users share the same table structure. Therefore the default "TEMPTABS" schema is considered sufficient for this emulation method.
The Global Temporary Tables are created with the "ON COMMIT PRESERVE ROWS" option, to keep the rows in the table when a transaction ends.
If the global temporary table to be created exists already, error ORA-00955 will be ignored by the database driver. This allows to do several CREATE TEMP TABLE statements in your programs with no SQL error, to emulate the Informix behavior. This works fine as long as the table name is unique for a given structure (column count and data types must match).
Note: Since DROP TABLE statements are converted to DELETE statements, you might want to use Oracle sqlplus to issue a real DROP TABLE of global temporary tables, to make a real cleanup from time to time.
By default, global temporary table is created without any tablespace option, and thus will be created in the default tablespace assigned to the current user.
If required, you can specify another tablespace name for Oracle tables with the following FGLPROFILE entry:
```
dbi.database.dsname.ora.temptables.tablespace = "mytemptabs"
```
Note: This tablespace must be a temporary tablespace.
Once the Global Temporary Table has been created, all other SQL statements performed in the current SQL session are parsed to convert the original table name to schema.original-tablename.
When doing a DROP TABLE temp-table statement in the program, the database driver converts it to a DELETE statement, to remove all data added by the current session. A next CREATE TEMP TABLE or SELECT INTO TEMP will fail with error ORA-00955 but since this error is ignored, it will be transparent for the program. We cannot use TRUNCATE TABLE because that would require at least DROP ANY TABLE privileges for all users.
When the BDL program disconnects from the database (for example, when it ends or when a CLOSE DATABASE instruction is executed), the tables that have not been dropped by the program with an explicit DROP TABLE statement will be automatically cleaned by Oracle.

Prerequisites when using the global emulation

You must create a database user (schema) dedicated to this emulation, the default name is "TEMPTABS":
```
CREATE USER temptabs IDENTIFIED BY pswd;
```
If your programs need to create temporary tables on the fly with this method, you must grant CREATE ANY TABLE + CREATE ANY INDEX system privilege to all DB users. This is not a good practice for security reasons. You better "prepare" the database by creating the Global Temporary Table (when using the default schema, create it with the TEMPTABS user). Do not forget to specify ON COMMIT PRESERVE ROWS option. Then grant INSERT, UPDATE, DELETE and SELECT object privileges to PUBLIC, for example:
```
CREATE GLOBAL TEMPORARY TABLE temptabs.mytable
  ( k INT PRIMARY KEY, c CHAR(10) ) ON COMMIT PRESERVE ROWS;
CREATE UNIQUE INDEX temptabs.ix1 ON temptabs.mytable ( c );
GRANT SELECT, UPDATE, INSERT, DELETE ON temptabs.mytable TO PUBLIC;
```
For testing purpose, consider using a user with DBA privileges, to simplify the configuration.

Limitations of the global emulation

Global Temporary Tables are shared by multiple users/sessions. In order to have the global emulation working properly, each temporary table name must be unique for a given table structure, for all programs. Avoid using temp tables names such as "tmp1". It is recommended to use table names as follows:
```
CREATE TEMP TABLE custinfo_1 (
    cust_id INTEGER,
    cust_name VARCHAR(50)
 );
CREATE TEMP TABLE custinfo_2 (
    cust_id INTEGER,
    cust_name VARCHAR(50),
    cust_addr VARCHAR(200)
 );
```
Tokens matching the original table names are converted to unique names in all SQL statements. Make sure you are not using the temp table name for other database objects, like columns. The following example illustrates this limitation:
```
CREATE TABLE tab1 ( key INTEGER, tmp1 CHAR(20) );
CREATE TEMP TABLE tmp1 ( col1 INTEGER, col2 CHAR(20) );
SELECT tmp1 FROM tab1 WHERE ... 
```

Creating indexes on temporary tables with global emulation

Indexes created on temporary tables get also the TEMPTABS schema prefix by default. Indexes created on temporary tables follow the same storage and schema settings as for the CREATE GLOBAL TEMPORARY TABLE statements: Schema and tablespace specification of FGLPROFILE will also be applied to CREATE INDEX commands.
When executing a DROP INDEX statement on a temporary table in a program, the database driver just ignores the statement.

SERIALs in temporary table creation with global emulation

You can use the SERIAL data type when creating a temporary table.
Sequences and triggers will be created in the TEMPTABS schema too.

See issue about SERIALs for more details.