Add Creating Custom Types as a postgres til

postgres/creating-custom-types.md

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+# Creating Custom Types
+
+PostgreSQL has support for creating custom types. When you need something
+more expressive than the built-in types and you don't want your data spread
+across multiple columns, you can instead create a custom type.
+
+```sql
+create type dimensions as (
+  width integer,
+  height integer,
+  depth integer
+);
+```
+
+This new type can then be used in the definition of a new table
+
+```sql
+create table moving_boxes (
+  id serial primary key,
+  dims dimensions not null
+);
+```
+
+and when inserting data
+
+```sql
+insert into moving_boxes (dims) values (row(3,4,5)::dimensions);
+```
+
+See the [`create type`
+docs](http://www.postgresql.org/docs/current/static/sql-createtype.html) for
+more details.