Add Use Rename To Hot Swap Two Tables 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).
 
-_1222 TILs and counting..._
+_1223 TILs and counting..._
 
 ---
 
@@ -678,6 +678,7 @@ _1222 TILs and counting..._
 - [Use A psqlrc File For Common Settings](postgres/use-a-psqlrc-file-for-common-settings.md)
 - [Use Argument Indexes](postgres/use-argument-indexes.md)
 - [Use Not Valid To Immediately Enforce A Constraint](postgres/use-not-valid-to-immediately-enforce-a-constraint.md)
+- [Use Rename To Hot Swap Two Tables](postgres/use-rename-to-hot-swap-two-tables.md)
 - [Using Expressions In Indexes](postgres/using-expressions-in-indexes.md)
 - [Using Intervals To Offset Time](postgres/using-intervals-to-offset-time.md)
 - [Who Is The Current User](postgres/who-is-the-current-user.md)

postgres/use-rename-to-hot-swap-two-tables.md

@@ -0,0 +1,34 @@
+# Use Rename To Hot Swap Two Tables
+
+The [`alter
+table`](https://www.postgresql.org/docs/current/sql-altertable.html) command
+can be used to [rename a
+table](https://www.postgresqltutorial.com/postgresql-tutorial/postgresql-rename-table/).
+Because it is changing the name of a reference rather than actually moving any
+data around, it is very fast.
+
+We can exploit the speed of a _rename_ to hot swap two tables. This is useful
+for a situation where we've created an identical table with a small fraction of
+the data of the original table. By hot swapping them, we've exchanged the large
+table for a smaller one without our application code noticing anything
+happened.
+
+Let's assume we have a massive `events` table and then a much smaller
+`new_events` table with the same structure.
+
+The following transaction will swap those two tables in the blink of an eye.
+
+```sql
+begin;
+
+alter table events rename to old_events;
+alter table new_events rename to events;
+
+commit;
+```
+
+The resulting `old_events` table can then be deleted at our convenience.
+
+The other nice thing about this approach is that, before deleting `old_events`,
+you can easily and quickly swap them back using the same approach. It is always
+a comfort when huge changes like this are easy to reverse if necessary.