fglhint_* SQL comments

Using special SQL comment hints to control statement execution.

Syntax

/*
{ fglhint_insert
| fglhint_update
| fglhint_delete
| fglhint_select
| fglhint_other
}
[ fglhint_no_ifxemul ]
*/

The fglhint_* indicators help the database driver to identify the SQL statement and control Informix® emulation:

  1. fglhint_insert: Regular INSERT (preparable, without result set).
  2. fglhint_update: Regular UPDATE (preparable, without result set).
  3. fglhint_delete: Regular DELETE (preparable, without result set).
  4. fglhint_select: Regular SELECT (preparable, with result set).
  5. fglhint_other: Any other non-preparable SQL statement (maybe with result set).
  6. fglhint_no_ifxemul: Disable Informix emulation (like the FGLPROFILE entry, but for this statement only).

Usage

When preparing an SQL statement, the ODI database drivers must identify the type of SQL statement, in order to set database client API options to perform the statement properly.

For example, when executing an SELECT statement that returns a result set, ODBC options for a server-side cursor must be set with Microsoft® SQL Server clients. If a server-side cursor is not used only one active cursor is used in a Genero program.

Or, for example, when executing an SELECT statement that creates temporary tables with SQL Server, it must be done with a direct ODBC execution (using ODBC SQLExecDirect instead of SQLPrepare/SQLExecute), otherwise the temporary table is created in the scope of the sp_prepare() stored procedure, and is dropped immediately after returning from the sp_prepare() call.

Specified inside /* */ C-style comments, the fglhint_* keywords can be used as SQL statement recognition directives:

  • When the SQL statement looks like a regular INSERT statement, but uses an OUTPUT INSERTED.* clause, add the /* fglhint_select */ SQL comment, to indicate that it must be treated as a regular, preparable SELECT statement returning a result set.
  • When the SQL statement looks like a regular SELECT statement, but has an INTO newtable clause to create a new table, add the /* fglhint_insert */ SQL comment, to indicate that it must be treated as a regular, preparable INSERT statement.
  • When the SQL statement looks like a regular SELECT statement, but has an INTO #newtemptable clause to create a new temporary table, add the /* fglhint_other */ SQL comment, to indicate that it must be treated as a non-preparable INSERT statement (to be executed directly).

/* */ C-style comments are only allowed in dynamic SQL statements, the BDL language does not allow such comments in static SQL statements, which is based on the Informix-SQL syntax.

For example, this statement performs an INSERT with an OUTPUT clause, that will produce a result set. Use the fglhint_select hint, to indicate that the statement must be executed as a regular SELECT:
DECLARE c1 CURSOR
   FROM "/* fglhint_select */ INSERT INTO table1 OUTPUT INSERTED.* SELECT * FROM customers"
This statement performs a SELECT INTO newtable, that will be executed as an INSERT (without a result set), because we use the fglhint_insert hint: 
EXECUTE IMMEDIATE "/* fglhint_insert */ SELECT * INTO newtable FROM customers WHERE cust_valid='Y'"
In order to perform a SELECT INTO #temptable, the statement must be executed directly without using the sp_prepare/sp_execute procedures of SQL Server (otherwise the temp table will only exist in the context of the stored procedures). Therefore, we use the fglhint_other hint to force a direct SQL execution: 
EXECUTE IMMEDIATE "/* fglhint_other */ SELECT * INTO #tmptab1 SELECT * FROM customers"
DECLARE c2 CURSOR FROM "SELECT * FROM #tmptab1 ..."
Furthermore, Informix SQL emulation can be disabled by using the fglhint_no_ifxemul hint:
PREPARE s1 FROM "/* fglhint_no_ifxemul */ SELECT * FROM [dbo].[mytable]"

When using the fglhint_no_ifxemul hint, Informix emulation will only be disabled for this SQL statement. Other SQL statements executed by your program can use the Informix emulations.

SQL statement type identification and Informix emulation hints can be combined: 
PREPARE s1 FROM "/* fglhint_no_ifxemul fglhint_select */ INSERT INTO [dbo].[mytable] OUTPUT ..."