Add Using Expressions In Indexes as a postgres til

postgres/using-expressions-in-indexes.md

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+# Using Expressions In Indexes
+
+Though we usually see column names by themselves when defining an index, it
+is also possible to create an index with an expression.
+
+Let's say I have a `users` table with an `email` column. Then I may end up
+creating an index like this
+
+```sql
+create index email_idx on users (email);
+```
+
+If I always perform queries on the `email` column with the `lower()`
+function, like this
+
+```sql
+select * from users where lower(email) = lower('some@email.com');
+```
+
+then I will want to also create an index with that full expression --
+`lower(email)`
+
+I can do this with a statement like the following
+
+```sql
+create index lower_email_idx on users (lower(email));
+```
+
+Without an index that uses the full `lower(email)` expression, `select`
+statements like the one above will be forced to do full sequential scans
+instead of indexed scans.