diff --git a/privatemsg.module b/privatemsg.module
index 32451e565d1af965b39175b031490a4f0035e47d..f0787dbea752a9321352873d1ebc0de481ecbf90 100644
--- a/privatemsg.module
+++ b/privatemsg.module
@@ -1004,21 +1004,29 @@ function pm_preview($form, &$form_state) {
 function privatemsg_sql_list(&$fragments, $account, $argument = 'list') {
   $fragments['primary_table'] = '{pm_message} pm';
 
-  // Load enabled columns
+  // Load enabled columns.
   $fields = array_filter(variable_get('privatemsg_display_fields', array('participants')));
 
-  // Required columns
+  // Required columns.
   $fragments['select'][]      = 'pmi.thread_id';
+  // We have to use MIN as the subject might not be the same in some threads.
+  // MIN() does not have a useful meaning except that it helps to correctly
+  // aggregate the thread on PostgreSQL.
   $fragments['select'][]      = 'MIN(pm.subject) as subject';
   $fragments['select'][]      = 'MAX(pm.timestamp) as last_updated';
+  // We use SUM so that we can count the number of unread messages.
   $fragments['select'][]      = 'SUM(pmi.is_new) as is_new';
 
+  // Select number of messages in the thread if the count is
+  // set to be displayed.
   if (in_array('count', $fields)) {
-    $fragments['select'][]      = 'COUNT(distinct pmi.mid) as count'; // We only want the distinct number of messages in this thread.
+    $fragments['select'][]      = 'COUNT(distinct pmi.mid) as count';
   }
   if (in_array('participants', $fields)) {
-    // Query for a string with uid's, for example "1,6,7". This needs a subquery on PostgreSQL.
+    // Query for a string with uid's, for example "1,6,7".
+    // @todo: Replace this with a single query similiar to the tag list.
     if ($GLOBALS['db_type'] == 'pgsql') {
+      // PostgreSQL does not know GROUP_CONCAT, so a subquery is required.
       $fragments['select'][]      = "array_to_string(array(SELECT DISTINCT textin(int4out(pmia.uid))
                                                             FROM {pm_index} pmia
                                                             WHERE pmia.thread_id = pmi.thread_id), ',') AS participants";
@@ -1032,8 +1040,10 @@ function privatemsg_sql_list(&$fragments, $account, $argument = 'list') {
   if (in_array('thread_started', $fields)) {
     $fragments['select'][]      = 'MIN(pm.timestamp) as thread_started';
   }
-  // pm_index needs to be the first join.
+
   $fragments['inner_join'][]  = 'INNER JOIN {pm_index} pmi ON pm.mid = pmi.mid';
+
+  // Only load undeleted messages of the current user and group by thread.
   $fragments['where'][]       = 'pmi.uid = %d';
   $fragments['query_args']['where'][]  = $account->uid;
   $fragments['where'][]       = 'pmi.deleted = 0';
@@ -1057,7 +1067,7 @@ function privatemsg_sql_list(&$fragments, $account, $argument = 'list') {
  *   Account for which the messages should be loaded.
  */
 function privatemsg_sql_load(&$fragments, $pmids, $account = NULL) {
-  $fragments['primary_table'] = '{pm_message} pm'; // Our primary table
+  $fragments['primary_table'] = '{pm_message} pm';
 
   $fragments['select'][]      = "pm.mid";
   $fragments['select'][]      = "pm.author";
@@ -1069,6 +1079,7 @@ function privatemsg_sql_load(&$fragments, $pmids, $account = NULL) {
   $fragments['select'][]      = "pmi.thread_id";
 
   $fragments['inner_join'][]  = 'INNER JOIN {pm_index} pmi ON pm.mid = pmi.mid';
+  // Use IN() to load multiple messages at the same time.
   $fragments['where'][]       = 'pmi.mid IN (' . db_placeholders($pmids) . ')';
   $fragments['query_args']['where']  += $pmids;
   if ($account) {
@@ -1093,6 +1104,8 @@ function privatemsg_sql_load(&$fragments, $pmids, $account = NULL) {
 function privatemsg_sql_messages(&$fragments, $threads, $account = NULL, $load_all = FALSE) {
   $fragments['primary_table'] = '{pm_index} pmi';
 
+  // Use DISTINCT to only load each mid once even if the user is listed more
+  // than once as recipient.
   $fragments['select'][]      = 'DISTINCT(pmi.mid) as mid';
   $fragments['where'][]       = 'pmi.thread_id IN ('. db_placeholders($threads) .')';
   $fragments['query_args']['where']   += $threads;
@@ -1103,6 +1116,7 @@ function privatemsg_sql_messages(&$fragments, $threads, $account = NULL, $load_a
     $fragments['query_args']['where'][]  = $account->uid;
   }
   if (!$load_all) {
+    // Also load deleted messages when requested.
     $fragments['where'][]       = 'pmi.deleted = 0';
   }
   $fragments['order_by'][]    = 'pm.timestamp ASC';
@@ -1119,6 +1133,8 @@ function privatemsg_sql_messages(&$fragments, $threads, $account = NULL, $load_a
 function privatemsg_sql_participants(&$fragments, $thread_id) {
   $fragments['primary_table'] = '{pm_index} pmi';
 
+  // Only load each participant once since they are listed as recipient for
+  // every message of that thread.
   $fragments['select'][]      = 'DISTINCT(pmi.uid) AS uid';
   $fragments['select'][]      = 'u.name AS name';
 
@@ -1127,26 +1143,48 @@ function privatemsg_sql_participants(&$fragments, $thread_id) {
   $fragments['query_args']['where'][]  = $thread_id;
 }
 
-
+/**
+ * Query definition to count unread messages.
+ *
+ * @param $fragments
+ *   Query fragments array.
+ * @param $account
+ *   User object for which the messages are being counted.
+ */
 function privatemsg_sql_unread_count(&$fragments, $account) {
   $fragments['primary_table'] = '{pm_index} pmi';
 
   $fragments['select'][]      = 'COUNT(DISTINCT thread_id) as unread_count';
+
+  // Only count new messages that have not been deleted.
   $fragments['where'][]       = 'pmi.deleted = 0';
   $fragments['where'][]       = 'pmi.is_new = 1';
   $fragments['where'][]       = 'pmi.uid = %d';
   $fragments['query_args']['where'][]  = $account->uid;
 }
 
+/**
+ * Query definition to search for username autocomplete suggestions.
+ *
+ * @param $fragments
+ *   Query fragments array.
+ * @param $search
+ *   Which search string is currently searched for.
+ * @param $names
+ *   Array of names not to be used as suggestions.
+ */
 function privatemsg_sql_autocomplete(&$fragments, $search, $names) {
   $fragments['primary_table'] = '{users} u';
   $fragments['select'][] = 'u.name';
+  // Escape the % to get it through the placeholder replacement.
   $fragments['where'][] = "u.name LIKE '%s'";
   $fragments['query_args']['where'][] = $search .'%%';
   if (!empty($names)) {
+    // If there are already names selected, exclude them from the suggestions.
     $fragments['where'][] = "u.name NOT IN (". db_placeholders($names, 'text') .")";
     $fragments['query_args']['where'] += $names;
   }
+  // Only load active users and sort them by name.
   $fragments['where'][] = 'u.status <> 0';
   $fragments['order_by'][] = 'u.name ASC';
 }
@@ -1163,14 +1201,19 @@ function privatemsg_sql_deleted(&$fragments, $days) {
   $fragments['primary_table'] = '{pm_message} pm';
 
   $fragments['select'][] = 'pm.mid';
+  // The lowest value is higher than 0 if all recipients have deleted a message.
   $fragments['select'][] = 'MIN(pmi.deleted) as is_deleted';
+  // The time the most recent deletion happened.
   $fragments['select'][] = 'MAX(pmi.deleted) as last_deleted';
 
   $fragments['inner_join'][] = 'INNER JOIN {pm_index} pmi ON (pmi.mid = pm.mid)';
 
   $fragments['group_by'][] = 'pm.mid';
 
+  // Ignore messages that have not been deleted by all users.
   $fragments['having'][] = 'MIN(pmi.deleted) > 0';
+
+  // Only select messages that have been deleted more than n days ago.
   $fragments['having'][] = 'MAX(pmi.deleted) < %d';
   $fragments['query_args']['having'][] = time() - $days * 86400;
 }
diff --git a/privatemsg_filter/privatemsg_filter.module b/privatemsg_filter/privatemsg_filter.module
index 3f0fd81a22d86017e70fe5b68a2451e5185b6fe8..d088800201aa1b32d1a374cb61f753fbe8626a3a 100644
--- a/privatemsg_filter/privatemsg_filter.module
+++ b/privatemsg_filter/privatemsg_filter.module
@@ -763,6 +763,11 @@ function privatemsg_filter_tags_autocomplete($string) {
   drupal_json((object)$matches);
 }
 
+/**
+ * @addtogroup sql
+ * @{
+ */
+
 /**
  * Limit the user autocomplete for the filter widget.
  *
@@ -775,8 +780,15 @@ function privatemsg_filter_tags_autocomplete($string) {
  */
 function privatemsg_filter_privatemsg_sql_autocomplete_alter(&$fragments, $search, $names) {
   global $user;
+  // arg(1) is an additional URL argument passed to the URL when only
+  // users that are listed as recipient for threads of that user should be
+  // displayed.
+  // @todo: Check if these results can be grouped to avoid unecessary loops.
   if (arg(1) == 'filter') {
+    // JOIN on index entries where the to be selected user is a recipient.
     $fragments['inner_join'][] = 'INNER JOIN {pm_index} pip ON pip.uid = u.uid';
+    // JOIN on rows where the current user is the recipient and that have the
+    // same mid as those above.
     $fragments['inner_join'][] = 'INNER JOIN {pm_index} piu ON piu.uid = %d AND pip.mid = piu.mid';
     $fragments['query_args']['join'][] = $user->uid;
   }
@@ -801,13 +813,16 @@ function privatemsg_filter_sql_tags(&$fragments, $user = NULL, $threads = NULL,
   $fragments['select'][] = 't.public';
 
   if (!empty($threads)) {
+    // If the tag list needs to be for specific threads.
     $fragments['select'][] = 'ti.thread_id';
     $fragments['inner_join'][]  = 'INNER JOIN {pm_tags_index} ti on ti.tag_id = t.tag_id';
     $fragments['where'][] = 'ti.thread_id IN (' . db_placeholders($threads) . ')';
     $fragments['query_args']['where'] += $threads;
   }
   else {
+    // Tag usage counter is only used when we select all tags.
     $fragments['select'][]      = 'COUNT(ti.thread_id) as count';
+    // LEFT JOIN so that unused tags are displayed too.
     $fragments['inner_join'][]  = 'LEFT JOIN {pm_tags_index} ti ON t.tag_id = ti.tag_id';
     $fragments['group_by'][]    = 't.tag_id';
     $fragments['group_by'][]    = 't.tag';
@@ -818,7 +833,6 @@ function privatemsg_filter_sql_tags(&$fragments, $user = NULL, $threads = NULL,
     $fragments['query_args']['where'][] = $user->uid;
   }
 
-
   // Only select n tags per thread (ordered per tag_id), see
   // http://www.xaprb.com/blog/2006/12/07/how-to-select-the-firstleastmax-row-per-group-in-sql/.
   //
@@ -835,7 +849,9 @@ function privatemsg_filter_sql_tags(&$fragments, $user = NULL, $threads = NULL,
                               AND pmtic.tag_id < ti.tag_id) < %d';
     $fragments['query_args']['where'][] = $limit;
   }
-  elseif (!empty($thread_id) || !empty($user)) {
+  elseif (!empty($threads) || !empty($user)) {
+    // Only add a sort when we are not loading the tags for the admin page.
+    // Sorting is handled through tablesort_sql() then.
     $fragments['order_by'][] = 't.tag ASC';
   }
 }
@@ -856,17 +872,26 @@ function privatemsg_filter_sql_tags_autocomplete(&$fragments, $search, $tags) {
   $fragments['primary_table'] = '{pm_tags} pmt';
   $fragments['select'][] = 'pmt.tag';
   $fragments['where'][] = "pmt.tag LIKE '%s'";
+  // Escape % to get through the placeholder replacement.
   $fragments['query_args']['where'][] = $search .'%%';
   if (!empty($tags)) {
+    // Exclude tags.
     $fragments['where'][] = "pmt.tag NOT IN (". db_placeholders($tags, 'text') .")";
     $fragments['query_args']['where'] += $tags;
   }
+  // LEFT JOIN to be able to load public, unused tags.
   $fragments['inner_join'][]  = 'LEFT JOIN {pm_tags_index} pmti ON pmt.tag_id = pmti.tag_id AND pmti.uid = %d';
   $fragments['query_args']['join'][] = $user->uid;
+  // Autocomplete should only display Tags used by that user or public tags.
+  // This is done to avoid information disclosure as part of tag names.
   $fragments['where'][] = '(pmti.uid IS NOT NULL OR pmt.public = 1)';
   $fragments['order_by'][] = 'pmt.tag ASC';
 }
 
+/**
+ * @}
+ */
+
 /**
  * Implement hook_user().
  */