Add Create An Index Across Two Columns as a Postgres TIL

postgres/create-an-index-across-two-columns.md

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+# Create An Index Across Two Columns
+
+Most commonly when we create an index, it is targeted at a single column of a
+table. Sometimes an expensive query that works with two different columns would
+be better off with an index that combines those two columns. This is called a
+_composite index_.
+
+Let's consider this query:
+
+```sql
+select * from events
+  where user_id = 123
+  order by created_at desc
+  limit 1;
+```
+
+Though this query will use the index on `created_at` to do an Index Scan, it
+will still have to do a bunch of expensive filtering of `user_id` values after
+the fact.
+
+What this query needs to be efficient is a _composite index_ on `user_id` and
+`created_at`. We can create one like so:
+
+```sql
+create index events_user_id_created_at_idx
+  on events (user_id, created_at);
+```
+
+Instead of doing a bunch of post-index filtering on `user_id` values, that
+expensive query will factor `user_id` into its Index Scan and complete much
+quicker.
+
+See [the Postgres docs on multicolumn
+indexes](https://www.postgresql.org/docs/current/indexes-multicolumn.html) for
+more details.