EXTFNAPIV4_DESCRIBE_COL_DISTINCT_VALUES (Get)

• EXTFNAPIV4_DESCRIBE_NOT_AVAILABLE – returned if the attribute was not available to get. This can happen if the column was not involved in the query.

On failure, returns one of the generic describe_column errors, or:

• EXTFNAPIV4_DESCRIBE_BUFFER_SIZE_MISMATCH – get error returned if the describe buffer is not the size of a_v4_extfn_estimate.
• EXTFNAPIV4_DESCRIBE_INVALID_STATE – get error returned if the specified argument is an input table and the query processing state is greater than Optimization.

CREATE PROCEDURE my_tpf( col_char char(10), col_table TABLE( c1 INT, c2 INT ) )
RESULTS ( r1 INT, r2 INT, r3 INT )
EXTERNAL ‘my_tpf_proc@mylibrary’;

CREATE TABLE T( x INT, y INT, z INT );

select * from my_tpf( 'test', TABLE( select x,y from T ) )

my_tpf_describe(a_v4_extfn_proc_context *cntxt)
{
    if( cntxt->current_state == EXTFNAPIV4_STATE_PLAN_BUILDING ) {
	       a_v4_extfn_estimate num_distinct;

	       a_sql_int32 ret = 0;

	       // Get the number of distinct values expected from the first column
        // of the table input parameter 'col_table'
	       ret = cntxt->describe_column_get( cntxt, 2, 1
		              EXTFNAPIV4_DESCRIBE_COL_DISTINCT_VALUES,
		              &num_distinct,
		              sizeof(a_v4_extfn_estimate) );

	       // default algorithm is 1
	       _algorithm = 1;

	       if( ret > 0 ) {
	           // choose the best algorithm for sample size.

	           if ( num_distinct.value < 100 ) {
		              // use faster algorithm for small distinct values.
	              	_algorithm = 2;
	           }
	       }
	       else {
	           if ( ret < 0 ) {
		              // Handle the error
		              //   or continue with default algorithm
	           } else {
		              // Attribute was unavailable
		              // We will use the default algorithm.
	           }
	       }
   }
}