From 03bd31aedd9ed8012eee7504aa5ef912623b0393 Mon Sep 17 00:00:00 2001
From: Helen Hou-Sandi <helen@git.wordpress.org>
Date: Fri, 20 Nov 2015 06:15:34 +0000
Subject: [PATCH] Custom fields: Allow for short-circuiting the meta key
 dropdown.

Adds the `postmeta_form_keys` filter which allows for a potentially expensive query against postmeta to be avoided.

props ericmann, tollmanz, nacin.
see #33885.


git-svn-id: https://develop.svn.wordpress.org/trunk@35717 602fd350-edb4-49c9-b593-d223f7449a82
---
 src/wp-admin/includes/template-functions.php | 39 ++++++++++++++------
 1 file changed, 27 insertions(+), 12 deletions(-)

diff --git a/src/wp-admin/includes/template-functions.php b/src/wp-admin/includes/template-functions.php
index ecfd48901c..f85e87a8bf 100644
--- a/src/wp-admin/includes/template-functions.php
+++ b/src/wp-admin/includes/template-functions.php
@@ -577,21 +577,36 @@ function meta_form( $post = null ) {
 	$post = get_post( $post );
 
 	/**
-	 * Filter the number of custom fields to retrieve for the drop-down
-	 * in the Custom Fields meta box.
+	 * Filter values for the meta key dropdown in the Custom Fields meta box.
 	 *
-	 * @since 2.1.0
+	 * Returning a non-null value will effectively short-circuit and avoid a
+	 * potentially expensive query against postmeta.
 	 *
-	 * @param int $limit Number of custom fields to retrieve. Default 30.
+	 * @since 4.4.0
+	 *
+	 * @param array|null $keys Pre-defined meta keys to be used in place of a postmeta query. Default null.
 	 */
-	$limit = apply_filters( 'postmeta_form_limit', 30 );
-	$sql = "SELECT DISTINCT meta_key
-		FROM $wpdb->postmeta
-		WHERE meta_key NOT BETWEEN '_' AND '_z'
-		HAVING meta_key NOT LIKE %s
-		ORDER BY meta_key
-		LIMIT %d";
-	$keys = $wpdb->get_col( $wpdb->prepare( $sql, $wpdb->esc_like( '_' ) . '%', $limit ) );
+	$keys = apply_filters( 'postmeta_form_keys', null );
+
+	if ( null === $keys ) {
+		/**
+		 * Filter the number of custom fields to retrieve for the drop-down
+		 * in the Custom Fields meta box.
+		 *
+		 * @since 2.1.0
+		 *
+		 * @param int $limit Number of custom fields to retrieve. Default 30.
+		 */
+		$limit = apply_filters( 'postmeta_form_limit', 30 );
+		$sql = "SELECT DISTINCT meta_key
+			FROM $wpdb->postmeta
+			WHERE meta_key NOT BETWEEN '_' AND '_z'
+			HAVING meta_key NOT LIKE %s
+			ORDER BY meta_key
+			LIMIT %d";
+		$keys = $wpdb->get_col( $wpdb->prepare( $sql, $wpdb->esc_like( '_' ) . '%', $limit ) );
+	}
+
 	if ( $keys ) {
 		natcasesort( $keys );
 		$meta_key_input_id = 'metakeyselect';