Add back lost Postgres creation sql for storage backend

website/source/docs/configuration/storage/postgresql.html.md

@@ -41,6 +41,36 @@ CREATE TABLE vault_kv_store (
 CREATE INDEX parent_path_idx ON vault_kv_store (parent_path);
 ```
 
+If you're using a version of PostgreSQL prior to 9.5, create the following function:
+
+```sql
+CREATE FUNCTION vault_kv_put(_parent_path TEXT, _path TEXT, _key TEXT, _value BYTEA) RETURNS VOID AS
+$$
+BEGIN
+    LOOP
+        -- first try to update the key
+        UPDATE vault_kv_store
+          SET (parent_path, path, key, value) = (_parent_path, _path, _key, _value)
+          WHERE _path = path AND key = _key;
+        IF found THEN
+            RETURN;
+        END IF;
+        -- not there, so try to insert the key
+        -- if someone else inserts the same key concurrently,
+        -- we could get a unique-key failure
+        BEGIN
+            INSERT INTO vault_kv_store (parent_path, path, key, value)
+              VALUES (_parent_path, _path, _key, _value);
+            RETURN;
+        EXCEPTION WHEN unique_violation THEN
+            -- Do nothing, and loop to try the UPDATE again.
+        END;
+    END LOOP;
+END;
+$$
+LANGUAGE plpgsql;
+```
+
 ## `postgresql` Parameters
 
 - `connection_url` `(string: <required>)` – Specifies the connection string to