Temporary tables

Informix®

Informix temporary tables are created with the CREATE TEMP TABLE DDL instruction or with SELECT ... INTO TEMP statement:

CREATE TEMP TABLE tt1 ( pkey INT, name VARCHAR(50) )
CREATE TEMP TABLE tt2 ( pkey INT, name VARCHAR(50) ) WITH NO LOG 
SELECT * FROM tab1 WHERE pkey > 100 INTO TEMP tt2

Temporary tables are automatically dropped when the SQL session ends, but they can also be dropped with the DROP TABLE command. There is no name conflict when several users create temporary tables with the same name.

BDL reports can create a temporary table when the rows are not sorted externally (by the source SQL statement).

Informix allows you to create indexes on temporary tables. No name conflict occurs when several users create an index on a temporary table by using the same index identifier.

When creating temporary tables in Informix, the WITH NO LOG clause can be used to avoid the overhead of recording DML operations in transaction logs.

PostgreSQL

PostgreSQL supports temporary table creation statements similar to Informix:

CREATE TEMP TABLE temptab ...

There is a little syntax difference in the SELECT INTO TEMP instruction. PostgreSQL uses the INTO TEMP clause after the SELECT list:

SELECT * INTO TEMP temptab FROM source

Solution

Temporary tables are well supported with native PostgreSQL temp tables.

Important:

Simple Informix-style SQL statement creating temporary tables can be converted to a native SQL equivalent instruction. However, complex SQL statements such as SELECT .. INTO TEMP with subqueries may fail. In such cases, create a view from the complex query and then create the temp table from the view. Or, disable Informix emulation and use the native SQL syntax to create the temporary table (EXECUTE IMMEDIATE "/* fglhint_no_ifxemul */ …")

With Informix SQL, if the source table has a column defined as SERIAL or BIGSERIAL, a SELECT ... INTO TEMP will produce a new temp table with an auto-incremented serial column. With the SELECT … INTO TEMP emulation for non-Informix databases, not using the native sequence generators (such as IDENTITY columns in SQL Server), the resulting temporary table will get a simple INTEGER or BIGINT column, instead of an auto-incremented column.

The general FGLPROFILE entry to control temporary table emulation is:

dbi.database.dsname.ifxemul.temptables = { true | false }

For more details see IBM Informix emulation parameters in FGLPROFILE.