Add Get A Quick Approximate Count Of A Large Table as a Rails TIL

rails/get-a-quick-approximate-count-of-a-large-table.md

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+# Get A Quick Approximate Count Of A Large Table
+
+Let's say our Rails app has a massive `events` table in it's Postgres database.
+We might be tempted to reach for an ActiveRecord API method like `Event.count`
+to get the number of records in the table. For tables with millions of rows,
+this is going to be slow.
+
+If all we need is an approximate count, there is a faster way that uses some of
+PostgreSQL's internal bookkeeping.
+
+We can request the approximate number of tuples recorded for our table by name.
+This query can be processed as raw SQL by the `#execute` method available on
+`ActiveRecord::Base.connection`.
+
+```ruby
+ActiveRecord::Base.connection.execute(<<~SQL)
+  select reltuples::numeric as count
+    from pg_class
+  where relname='events';
+SQL
+```
+
+That is going to spit out the `PG::Result` object which doesn't look like much
+on its own.
+
+```
+#<PG::Result:0x00 ...>
+```
+
+If we tack on a couple other methods, we can get the count as our result.
+
+```ruby
+ActiveRecord::Base.connection.execute(<<~SQL).to_a.first["count"].to_i
+  select reltuples::numeric as count
+    from pg_class
+  where relname='events';
+SQL
+```