Can somebody explain this weird error in pig

Question

I am using macros in pig , however it throws me an error saying : undefined alias 'result_1'.

The macro i am using :

 define macro_result (source , metric_name , metric_value) returns  result_metric 
        {
                result_1= foreach $source generate
                        timestamp,
                        member_sk as id,
                        '$metric_name' as minor_metric,
                        'Lts_seo' as major_metric,
                        $metric_value as value;
                $result_metric = result_1;

        };

I am calling the macro as :

page_views_to_jserp_from_job_detail    = macro_result(metadata_final,'PAGE_VIEWS_TO_JSERP_FROM_JOB_DETAIL', PV_to_jserp_from_job_detail  );

I changed the macro to this and the error seems to be resolved (change bolded) :

 define macro_result (source , metric_name , metric_value) returns  result_metric 
            {
                    ***$result_metric***= foreach $source generate
                            timestamp,
                            member_sk as id,
                            '$metric_name' as minor_metric,
                            'Lts_seo' as major_metric,
                            $metric_value as value;
            ***--       $result_metric = result_1;***

            };

What was causing the error initially? why can't i use result_1 as an intermediate step and store it in result_metric inside the macro?

Answer 1

We can NOT assign one alias directly to another.

We can project the required fields from one alias and return the same.

For the use case shared :

$result_metric = FOREACH result_1 GENERATE *;

This step is not required unless there is a change in projected fields between these two aliases.