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

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

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+# 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)