Support Plugins for Views Aggregate in D7 (D8 Backport)

Problem/Motivation

Views supports a minimal set of aggregate functions that are hard coded into the default query plugin. It would be useful to have a way to add support for the full range of SQL aggregates available in MySQL & PostgreSQL (GROUP_CONCT/STRING_AGG, VARIANCE, ...), as well as providing a means of supporting the extended set of OGC spatial aggregates supported by PostgreSQL (ST_Union, ST_ClusterWithin, ...), and allowing any other user-defined aggregates that may be available.

Proposed resolution

• Provide a plugin for defining new aggregates.
• Use CTools-based methods to expose plugin.
• This is a backport of a proposed solution for D8 (Support Plugins for Views Aggregate (2662548))

Code 1: Patch excerpt, 1 line to add aggregate definitions via plugin.

--- a/plugins/views_plugin_query_default.inc
+++ b/plugins/views_plugin_query_default.inc
@@ -1592,7 +1592,7 @@ class views_plugin_query_default extends views_plugin_query {
           'sort' => 'views_handler_sort_group_by_numeric',
         ),
       )
-    );
+    ) + views_fetch_plugin_data('query_aggregate');
   }
 
   /**

For further details, see "Implementation Example", and for further details a related issue with patch for implementation in D7 "Support Plugins for Views Aggregate in D7").

Remaining tasks

1. Add ctools plugin search in aggregate definition function
2. Create Plugins for all default aggregate functions in order to provide implementation example.
3. Remove hard-coded default aggregates from views_plugin_query_default.inc
4. Test

User interface changes

None.

API changes

Add Annotation-based plugin support for ViewsAggregate

Data model changes

None.

Implementation Example

Code 2: Sample implementation of VAR_SAMP in Drupal 7 using Ctools plugin.

function views_views_plugins() {
  return array(
    'query_aggregate' => array(
      'var_samp' => array(
        'title' => t('Sample Variance'),
        'method' => 'views_query_default_aggregation_method_simple',
        'handler' => array(
          'argument' => 'views_handler_argument_group_by_numeric',
          'filter' => 'views_handler_filter_group_by_numeric',
          'sort' => 'views_handler_sort_group_by_numeric',
        ),
      ),
...

Original report by joachim

  function get_aggregation_info() {
    // @todo -- need a way to get database specific and customized aggregation
    // functions into here.

What's the best way to do this?

Should aggregation functions be another type of plugin?

(Background for this is that for Commerce product node displays it would be useful to have a GROUP_CONCAT available.)