:db_condition_placeholder leads to cache misses and superfluous cache_set

So this issue should be postponed for now.

Could this be related to http://drupal.org/node/1430650#comment-6961132 ?

On the first call, the DB placeholders for the output cache are _16, _17, _18, etc.

On the second call, when the results cache is active, the placeholders are _1, _2, etc.

This leads to a cache miss on output cache on the second page load and an unreachable cached item.

The approach in #1430650: Taxonomy filter with depth, Drupal 7.12, and views query caching failure fixes it in theory, but relies on internal DB functionality.

A better approach is:

diff --git a/docroot/sites/all/modules/contrib/views/plugins/views_plugin_cache.inc b/docroot/sites/all/modules/contrib/views/plugins/views_plugin_cache.inc
index 76bb4c66f..2a0b6b0e4 100644
--- a/docroot/sites/all/modules/contrib/views/plugins/views_plugin_cache.inc
+++ b/docroot/sites/all/modules/contrib/views/plugins/views_plugin_cache.inc
@@ -334,9 +334,14 @@ class views_plugin_cache extends views_plugin {
         if ($build_info[$index] instanceof SelectQueryInterface) {
           $query = clone $build_info[$index];
           $query->preExecute();
+
+          // Ensure that placeholders are always numerated from 0 to n.
+          $sql = (string) $query;
+          $arguments = (array) $query->getArguments();
+
+          $cache_arguments = array();
+          foreach (array_keys($arguments) as $key => $value) {
+            $cache_arguments[$value] = ':cache_placeholder_' . $key;
+          }
+
           $key_data['build_info'][$index] = array(
-            'sql' => (string) $query,
-            'arguments' => $query->getArguments(),
+            'sql' => strtr($sql, $cache_arguments),
+            'arguments' => array_values($arguments),
           );
         }
       }

Ohhhhh, I finally understand what is happening (after speaking with Earl about it!) and there is a better fix:

Pseudo-Code:

  $query = $build_info[$index];
  if (!$query->prepared()) {
    $query = clone $build_info[$index];
    $query->preExecute();
  }

should be all that is needed.

The reason why it works for results cache, but fails for output cache is because output cache is actually _after_ execution of the query, which is why the presence or absence of an already executed query makes it fail with different placeholders.

It works!

diff --git a/docroot/sites/all/modules/contrib/views/plugins/views_plugin_cache.inc b/docroot/sites/all/modules/contrib/views/plugins/views_plugin_cache.inc
index 76bb4c66f..67da0b9eb 100644
--- a/docroot/sites/all/modules/contrib/views/plugins/views_plugin_cache.inc
+++ b/docroot/sites/all/modules/contrib/views/plugins/views_plugin_cache.inc
@@ -332,8 +332,15 @@ class views_plugin_cache extends views_plugin {
         // If the default query back-end is used generate SQL query strings from
         // the query objects.
         if ($build_info[$index] instanceof SelectQueryInterface) {
-          $query = clone $build_info[$index];
-          $query->preExecute();
+          $query = $build_info[$index];
+
+          // If the query was not yet prepared, work on a clone and run
+          // preExecute().
+          if (!$query->isPrepared()) {
+            $query = clone $build_info[$index];
+            $query->preExecute();
+          }
+
           $key_data['build_info'][$index] = array(
             'sql' => (string) $query,
             'arguments' => $query->getArguments(),

And here is the patch.

Excellent work, thank you Fabianx!

This is indeed a nice fix!

This looks perfect from my point of view.

Committed. Thanks!