FEATURE: Tweak postgres_highest_sequence only for int columns (#98)

Tweak `postgres_highest_sequence` metric to export the highest sequence
only for integer columns. These are the most probable columns to run out
of values.

There are some exceptions to this rule to include `bigint` columns that
may be referenced in `integer` columns. For example, `reviewables.id` is
a `bigint` column, but `reviewable_scores.reviewable_id` is an integer.

Author: nbianca

lib/internal_metric/global.rb

@@ -368,10 +368,25 @@ def calc_postgres_highest_sequence
 
       RailsMultisite::ConnectionManagement.each_connection do |db|
         result[{ db: db }] = DB.query_single(<<~SQL)[0]
-          SELECT last_value
-          FROM pg_sequences
-          ORDER BY last_value DESC NULLS LAST
-          LIMIT 1
+          WITH columns_and_sequences AS (
+            SELECT table_name,
+                   column_name,
+                   information_schema.columns.data_type column_type,
+                   pg_sequences.data_type::text sequence_type,
+                   COALESCE(last_value, 0) last_value
+            FROM information_schema.columns
+            JOIN pg_sequences ON sequencename = REPLACE(REPLACE(column_default, 'nextval(''', ''), '''::regclass)', '')
+            WHERE information_schema.columns.table_schema = 'public'
+          )
+          SELECT MAX(last_value)
+          FROM columns_and_sequences
+          WHERE column_type = 'integer' OR
+                -- The column and sequence types should match, but this is just an extra check.
+                sequence_type = 'integer' OR
+                -- The `id` column of these tables is a `bigint`, but the foreign key columns are usually integers.
+                -- These columns will be migrated in the future.
+                -- See https://github.com/discourse/discourse/blob/6e1aeb1f504f469ceed189c24d43a7a99b8970c7/spec/rails_helper.rb#L480-L490
+                table_name IN ('reviewables', 'flags', 'sidebar_sections')
         SQL
       end
 

spec/lib/internal_metric/global_spec.rb

@@ -110,7 +110,7 @@
     expect(metric.postgres_highest_sequence).to be_a_kind_of(Hash)
     expect(metric.postgres_highest_sequence[{ db: "default" }]).to be_present
 
-    expect do metric.collect end.not_to change {
+    expect { metric.collect }.not_to change {
       metric.class.class_variable_get(:@@postgres_highest_sequence_last_check)
     }
   end