Add Postgre Does Not Support Unsigned Integers as a PostgreSQL TIL

postgres/postgres-does-not-support-unsigned-integers.md

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+# Postgres Does Not Support Unsigned Integers
+
+PostgreSQL has a variety of sizes of integer types, from `smallint` (2 bytes)
+to `integer` (4 bytes) to `bigint` (8 bytes), as well as [other numeric
+types](https://www.postgresql.org/docs/current/datatype-numeric.html).
+
+It does _not_ however support unsigned versions of these numeric types.
+
+That means, with an `integer` for instance, we can store numbers between
+`-2147483648` and `+2147483647`. That's everything that can fit into 4 bytes.
+In a system that supported 4 byte unsigned integers we'd be able to represent
+from `0` all the way up to `4294967295`.
+
+In PostgreSQL, we're limited to these _signed_ numeric types.
+
+That means if we were hoping that the data type could essentially enforce a
+non-negative restriction on the data in one of our columns, we're going to have
+to be more creative. The obvious choice to me is to consider adding a [check
+constraint](https://www.postgresql.org/docs/current/ddl-constraints.html#DDL-CONSTRAINTS-CHECK-CONSTRAINTS)
+(e.g. `quantity integer check (quantity > 0)`).
+
+Another option, as pointed out by [this StackOverflow
+answer](https://stackoverflow.com/a/31833279/535590), is to create [a
+user-defined _domain
+type_](https://www.postgresql.org/docs/current/domains.html) that restricts
+valid values. To me, the ergonomics of using a domain type are a bit awkward
+and not worth the effort.
+
+With either of these solutions, we are only approximating an unsigned integer
+and do not actually have the same range of values available.