[DATABASE] Add explicit indices for all foreign keys

This adds a requirement for all definitions that have foreign keys to also
require indices for all source (local) attributes mentioned in foreign keys.

MariaDB/MySQL creates indices for source attributes automatically, so this
serves as a way to get rid of those automatic indices and create clean explicit
ones instead.

In PostgreSQL, most of the time, indices on the source are necessary to
decrease performance penalty of foreign keys (like in MariaDB), but they aren't
created automatically, so this serves to remove that difference between
PostgreSQL and MariaDB.

lib/database/schema.php

@@ -1083,8 +1083,44 @@ class Schema
         }
 
         // A few quick checks :D
-        if (!isset($def['fields'])) {
-            throw new Exception("Invalid table definition for $tableName: no fields.");
+        if (!array_key_exists('fields', $def)) {
+            throw new ServerException(
+                "Invalid table definition for {$tableName}: no fields."
+            );
+        }
+
+        // Invalidate foreign key definitions that lack a specified index
+        if (array_key_exists('foreign keys', $def)) {
+            foreach ($def['foreign keys'] as $fkey_name => $fkey_def) {
+                $fkey_cols = array_keys($fkey_def[1]);
+
+                // A list of all keys/indices
+                $keys = array_values(array_merge(
+                    ($def['unique keys'] ?? []),
+                    ($def['indexes'] ?? [])
+                ));
+                if (array_key_exists('primary key', $def)) {
+                    $keys[] = $def['primary key'];
+                }
+
+                $indexed = false;
+                foreach ($keys as $key_cols) {
+                    // Only the beginning of a key counts
+                    $cols = array_slice($key_cols, 0, count($fkey_cols));
+
+                    if ($cols == $fkey_cols) {
+                        $indexed = true;
+                        break;
+                    }
+                }
+
+                if (!$indexed) {
+                    throw new ServerException(
+                        "Invalid table definition for {$tableName}: "
+                        . "foreign key {$fkey_name} is not indexed."
+                    );
+                }
+            }
         }
 
         return $def;