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

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1285 TILs and counting..._
+_1286 TILs and counting..._
 
 ---
 
@@ -797,6 +797,7 @@ _1285 TILs and counting..._
 - [Find Records With Multiple Associated Records](rails/find-records-with-multiple-associated-records.md)
 - [Force All Users To Sign Out](rails/force-all-users-to-sign-out.md)
 - [Generating And Executing SQL](rails/generating-and-executing-sql.md)
+- [Get A Quick Approximate Count Of A Large Table](rails/get-a-quick-approximate-count-of-a-large-table.md)
 - [Get ActiveRecord Attribute Directly From Database](rails/get-active-record-attribute-directly-from-database.md)
 - [Get An Array Of Values From The Database](rails/get-an-array-of-values-from-the-database.md)
 - [Get An Empty ActiveRecord Relation](rails/get-an-empty-activerecord-relation.md)

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

@@ -0,0 +1,38 @@
+# 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
+```