Add Include Columns In A Covering Index as a Postgres TIL

postgres/include-columns-in-a-covering-index.md

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+# Include Columns In A Covering Index
+
+A _covering index_ is a special type of B-Tree index that, in addition to
+indexing on a certain field, also _includes_ one or more columns as extra data
+in the leaves of the tree. When created correctly, this can speed up the
+queries it targets by achieving an _index-only scan_.
+
+Let's say we have a frequently run query on a large `events` table that looks
+like this:
+
+```sql
+select user_id, identifier, type
+  from events
+  where user_id = $1;
+```
+
+Here is what it looks like to create an index for this query with the `include`
+keyword:
+
+```sql
+create index user_id_on_events_idx
+  on (user_id)
+  includes (identifier, type);
+```
+
+An index on its own can already cause a significant speed up to the queries it
+targets, but may still need to retrieve some `select` attributes from the
+table. For hot-path queries with a set of specific columns always included in
+the select, there can be significant additional speed ups by having the index
+_cover_ those columns.
+
+For more details, check out [A Close Look At The Index Include
+Clause](https://use-the-index-luke.com/blog/2019-04/include-columns-in-btree-indexes).