[jira] [Created] (SOLR-7707) Add StreamExpression Support to RollupStream

["Dennis Gove (JIRA)" <ji...@apache.org>]

Dennis Gove created SOLR-7707:
---------------------------------

             Summary: Add StreamExpression Support to RollupStream
                 Key: SOLR-7707
                 URL: https://issues.apache.org/jira/browse/SOLR-7707
             Project: Solr
          Issue Type: Improvement
          Components: SolrJ
            Reporter: Dennis Gove
            Priority: Minor


This ticket is to add Stream Expression support to the RollupStream as discussed in SOLR-7560.

Proposed expression syntax for the RollupStream (copied from that ticket)

{code}
rollup(
  someStream(....),
  over="fieldA, fieldB, fieldC",
  min(fieldA),
  max(fieldA),
  min(fieldB),
  mean(fieldD),
  sum(fieldC)
)
{code}

This requires making the *Metric types Expressible but I think that ends up as a good thing. Would make it real easy to support other options on metrics like excluding outliers, for example find the sum of values within 3 standard deviations from the mean could be 
{code}
sum(fieldC, limit=standardDev(3))
{code}
 (note, how that particular calculation could be implemented is left as an exercise for the reader, I'm just using it as an example of adding additional options on a relatively simple metric).
Another option example is what to do with null values. For example, in some cases a null should not impact a mean but in others it should. You could express those as
{code}
mean(fieldA, replace(null, 0))  // replace null values with 0 thus leading to an impact on the mean
mean(fieldA, includeNull="true") // nulls are counted in the denominator but nothing added to numerator
mean(fieldA, includeNull="false") // nulls neither counted in denominator nor added to numerator
mean(fieldA, replace(null, fieldB), includeNull="true") // if fieldA is null replace it with fieldB, include null fieldB in mean
{code}
so on and so forth.



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