CHANGELOG.md: Add note about fl parameter

When indexing item views and downloads the only field we need is the
the id. The `fl` parameter tells Solr which fields to return in the
search results. This should theoretically be more efficient, though
I don't have any time to figure out how to measure it right now.

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 - Add ORDER BY to /items resource to make sure results are returned
 deterministically
+- Use `fl` parameter in indexer to return only the id field
 
 ## [1.3.2] - 2020-11-18
 ### Fixed

dspace_statistics_api/indexer.py

@@ -47,6 +47,7 @@ def index_views():
     solr_query_params = {
         "q": "type:2",
         "fq": "-isBot:true AND statistics_type:view",
+        "fl": "id",
         "facet": "true",
         "facet.field": "id",
         "facet.mincount": 1,
@@ -93,6 +94,7 @@ def index_views():
                 solr_query_params = {
                     "q": "type:2",
                     "fq": "-isBot:true AND statistics_type:view",
+                    "fl": "id",
                     "facet": "true",
                     "facet.field": "id",
                     "facet.mincount": 1,