Add Use Rename To Hot Swap Two Tables as a Postgres TIL

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

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+# 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.