Count all issuable states at once

Instead of doing n queries for n states, do one query to get all the
counts grouped by state, and figure out what the count is for each state
is from that. We can still cache the individual counts (it can't hurt),
but this will help with initial load.

Note that the `opened` scope on `Issuable` includes the `opened` and
`reopened` states, which is why there's a special case.

app/finders/issuable_finder.rb

@@ -49,6 +49,32 @@ class IssuableFinder
     execute.find_by(*params)
   end
 
+  # We often get counts for each state by running a query per state, and
+  # counting those results. This is typically slower than running one query
+  # (even if that query is slower than any of the individual state queries) and
+  # grouping and counting within that query.
+  #
+  def count_by_state
+    count_params = params.merge(state: nil, sort: nil)
+    labels_count = label_names.any? ? label_names.count : 1
+    finder = self.class.new(current_user, count_params)
+    counts = Hash.new(0)
+
+    # Searching by label includes a GROUP BY in the query, but ours will be last
+    # because it is added last. Searching by multiple labels also includes a row
+    # per issuable, so we have to count those in Ruby - which is bad, but still
+    # better than performing multiple queries.
+    #
+    finder.execute.reorder(nil).group(:state).count.each do |key, value|
+      counts[Array(key).last.to_sym] += value / labels_count
+    end
+
+    counts[:all] = counts.values.sum
+    counts[:opened] += counts[:reopened]
+
+    counts
+  end
+
   def group
     return @group if defined?(@group)
 

app/helpers/issuables_helper.rb

@@ -180,12 +180,9 @@ module IssuablesHelper
   end
 
   def issuables_count_for_state(issuable_type, state)
-    issuables_finder = public_send("#{issuable_type}_finder")
-
-    params = issuables_finder.params.merge(state: state)
-    finder = issuables_finder.class.new(issuables_finder.current_user, params)
-
-    finder.execute.page(1).total_count
+    @counts ||= {}
+    @counts[issuable_type] ||= public_send("#{issuable_type}_finder").count_by_state
+    @counts[issuable_type][state]
   end
 
   IRRELEVANT_PARAMS_FOR_CACHE_KEY = %i[utf8 sort page]