From 05a5af48bed2ce3044a1769c5e197c13d1369d89 Mon Sep 17 00:00:00 2001
From: jbranchaud <jbranchaud@gmail.com>
Date: Tue, 19 Apr 2016 22:01:24 -0500
Subject: [PATCH] Add Using Expressions In Indexes as a postgres til

---
 README.md                                |  3 ++-
 postgres/using-expressions-in-indexes.md | 31 ++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)
 create mode 100644 postgres/using-expressions-in-indexes.md

diff --git a/README.md b/README.md
index 8a3136d..86e068b 100644
--- a/README.md
+++ b/README.md
@@ -7,7 +7,7 @@ variety of languages and technologies. These are things that don't really
 warrant a full blog post. These are mostly things I learn by pairing with
 smart people at [Hashrocket](http://hashrocket.com/).
 
-_396 TILs and counting..._
+_397 TILs and counting..._
 
 ---
 
@@ -237,6 +237,7 @@ _396 TILs and counting..._
 - [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)
diff --git a/postgres/using-expressions-in-indexes.md b/postgres/using-expressions-in-indexes.md
new file mode 100644
index 0000000..3947767
--- /dev/null
+++ b/postgres/using-expressions-in-indexes.md
@@ -0,0 +1,31 @@
+# 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.