Add Using Expressions In Indexes as a postgres til

README.md

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 warrant a full blog post. These are mostly things I learn by pairing with
 smart people at [Hashrocket](http://hashrocket.com/).
 
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 - [Types By Category](postgres/types-by-category.md)
 - [Use A psqlrc File For Common Settings](postgres/use-a-psqlrc-file-for-common-settings.md)
 - [Use Argument Indexes](postgres/use-argument-indexes.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)
 - [Word Count for a Column](postgres/word-count-for-a-column.md)

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.