Add Get A Quick Approximate Count Of A Table as a Postgres til

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).
 
-_1151 TILs and counting..._
+_1152 TILs and counting..._
 
 ---
 
@@ -572,6 +572,7 @@ _1151 TILs and counting..._
 - [Generate Random UUIDs Without An Extension](postgres/generate-random-uuids-without-an-extension.md)
 - [Generate Series Of Numbers](postgres/generate-series-of-numbers.md)
 - [Generating UUIDs With pgcrypto](postgres/generating-uuids-with-pgcrypto.md)
+- [Get A Quick Approximate Count Of A Table](postgres/get-a-quick-approximate-count-of-a-table.md)
 - [Get The Size Of A Database](postgres/get-the-size-of-a-database.md)
 - [Get The Size Of A Table](postgres/get-the-size-of-a-table.md)
 - [Get The Size Of An Index](postgres/get-the-size-of-an-index.md)

postgres/get-a-quick-approximate-count-of-a-table.md

@@ -0,0 +1,42 @@
+# Get A Quick Approximate Count Of A Table
+
+Really large PostgreSQL tables can be slow to work with. Even a count of the
+rows in a really large table can take a while to tabulate. I'm talking about
+tables on the order of hundreds of millions of rows.
+
+For instance, here is a query grabbing the count of a ~400 million row table.
+
+```sql
+> select count(*) from events;
+
+   count
+-----------
+ 427462316
+(1 row)
+
+Time: 55113.794 ms
+```
+
+If I'm willing to wait nearly a minute (55 seconds), I can get an accurate
+count of the rows in this `events` table.
+
+If I don't want to wait and an approximate count will do, there are faster
+ways. One way is to query the `pg_class` table.
+
+```
+> select reltuples::numeric as count
+  from pg_class
+  where relname='events';
+
+   count
+-----------
+ 427462000
+(1 row)
+
+Time: 0.413 ms
+```
+
+The resulting count is within hundreds of the actual value and tells me what I
+need to know. And instead of 55 seconds, it takes less than half a millisecond.
+
+[source](https://andyatkinson.com/postgresql-tips)