Reuse Korma DB connection pools instead of creating new ones all the time

src/metabase/models/database.clj

@@ -27,16 +27,27 @@
    This will reuse `*jdbc-metadata*` if it's already set (to avoid opening extra connections).
    Otherwise it will open a new metadata connection and bind `*jdbc-metadata*` so it's available in subsequent calls to `with-jdbc-metadata` within F."
   [{:keys [connection]} f]
+  {:pre [(delay? connection)]}
   (if *jdbc-metadata* (f *jdbc-metadata*)
       (jdbc/with-db-metadata [md @connection]
         (binding [*jdbc-metadata* md]
           (f *jdbc-metadata*)))))
 
+;; Cache the Korma DB connections for given Database
+;; instead of creating new ones every single time
+(def ^:private connection->korma-db
+  (memoize
+   (fn [connection]
+     {:pre [(map? connection)]}
+     (log/debug "CREATING A NEW DB CONNECTION...")
+     (kdb/create-db connection))))
+
 (defn korma-db
   "Return a Korma database definition for DATABASE."
   [{:keys [connection]}]
+  {:pre [(delay? connection)]}
   (log/debug "CREATING A NEW DB CONNECTION...")
-  (kdb/create-db @connection))
+  (connection->korma-db @connection))
 
 (defn table-names
   "Fetch a list of table names for DATABASE."