Add Default Schema as a postgres til.

postgres/default-schema.md

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+# Default Schema
+
+Schemas can be used to organize tables within a database. You can see all
+the schemas your database has like so
+
+```sql
+> select schema_name from information_schema.schemata;
+    schema_name
+--------------------
+ pg_toast
+ pg_temp_1
+ pg_toast_temp_1
+ pg_catalog
+ public
+ information_schema
+(6 rows)
+```
+
+When you create a new table, it will need to be placed under one of these
+schemas. So if you have a `create table posts (...)`, how does postgres know
+what schema to put it under?
+
+Postgres checks your `search_path` for a default.
+
+```sql
+> show search_path;
+   search_path
+-----------------
+ "$user", public
+(1 row)
+```
+
+From our first select statement, we see that there is no schema with my user
+name, so postgres uses public as the default schema.
+
+If we set the search path to something that won't resolve to a schema name,
+postgres will complain
+
+```sql
+> set search_path = '$user';
+SET
+> create table posts (...);
+ERROR:  no schema has been selected to create in
+```