#633678 by Josh Waihi, chx, Crell, David Strauss, and Damien Tournoud: Make sequence API work on non-MySQL databases.

includes/database/database.inc

@@ -1178,26 +1178,7 @@ abstract class DatabaseConnection extends PDO {
    *   An integer number larger than any number returned by earlier calls and
    *   also larger than the $existing_id if one was passed in.
    */
-  public function nextId($existing_id = 0) {
+  abstract public function nextId($existing_id = 0);
-    $transaction = $this->startTransaction();
-    // We can safely use literal queries here instead of the slower query
-    // builder because if a given database breaks here then it can simply
-    // override nextId. However, this is unlikely as we deal with short strings
-    // and integers and no known databases require special handling for those
-    // simple cases. If another transaction wants to write the same row, it will
-    // wait until this transaction commits.
-    $stmt = $this->query('UPDATE {sequences} SET value = GREATEST(value, :existing_id) + 1', array(
-      ':existing_id' => $existing_id,
-    ));
-    if (!$stmt->rowCount()) {
-      $this->query('INSERT INTO {sequences} (value) VALUES (:existing_id + 1)', array(
-        ':existing_id' => $existing_id,
-      ));
-    }
-    // The transaction gets committed when the transaction object gets destroyed
-    // because it gets out of scope.
-    return $new_value;
-  }
 }
 
 /**

includes/database/pgsql/database.inc

@@ -11,6 +11,11 @@
  * @{
  */
 
+/**
+ * The name by which to obtain a lock for retrive the next insert id.
+ */
+define('POSTGRESQL_NEXTID_LOCK', 1000);
+
 class DatabaseConnection_pgsql extends DatabaseConnection {
 
   public function __construct(array $connection_options = array()) {
@@ -127,6 +132,49 @@ class DatabaseConnection_pgsql extends DatabaseConnection {
 
     return isset($specials[$operator]) ? $specials[$operator] : NULL;
   }
+
+  /**
+   * Retrive a the next id in a sequence.
+   *
+   * PostgreSQL has built in sequences. We'll use these instead of inserting
+   * and updating a sequences table.
+   */
+  public function nextId($existing = 0) {
+
+    // Retrive the name of the sequence. This information cannot be cached 
+    // because the prefix may change, for example, like it does in simpletests.
+    $sequence_name = $this->makeSequenceName('sequences', 'value');
+
+    // When PostgreSQL gets a value too small then it will lock the table, 
+    // retry the INSERT and if it's still too small then alter the sequence. 
+    $id = $this->query("SELECT nextval('" . $sequence_name . "')")->fetchField();
+    if ($id > $existing) {
+      return $id;
+    }
+
+    // PostgreSQL advisory locks are simply locks to be used by an
+    // application such as Drupal. This will prevent other Drupal proccesses
+    // from altering the sequence while we are. 
+    $this->query("SELECT pg_advisory_lock(" . POSTGRESQL_NEXTID_LOCK . ")");
+
+    // While waiting to obtain the lock, the sequence may have been altered
+    // so lets try again to obtain an adequate value.
+    $id = $this->query("SELECT nextval('" . $sequence_name . "')")->fetchField();
+    if ($id > $existing) {
+      $this->query("SELECT pg_advisory_unlock(" . POSTGRESQL_NEXTID_LOCK . ")");
+      return $id;
+    }
+
+    // Reset the sequence to a higher value than the existing id.
+    $this->query("ALTER SEQUENCE " . $sequence_name . " RESTART WITH " . ($existing + 1));
+
+    // Retrive the next id. We know this will be as high as we want it.
+    $id = $this->query("SELECT nextval('" . $sequence_name . "')")->fetchField();
+
+    $this->query("SELECT pg_advisory_unlock(" . POSTGRESQL_NEXTID_LOCK . ")");
+
+    return $id;
+  }
 }
 
 /**

includes/database/sqlite/database.inc

@@ -167,6 +167,27 @@ class DatabaseConnection_sqlite extends DatabaseConnection {
     // DatabaseStatement_sqlite::execute() and cannot be cached.
     return $this->prepare($this->prefixTables($query));
   }
+
+  public function nextId($existing_id = 0) {
+    $transaction = $this->startTransaction();
+    // We can safely use literal queries here instead of the slower query
+    // builder because if a given database breaks here then it can simply
+    // override nextId. However, this is unlikely as we deal with short strings
+    // and integers and no known databases require special handling for those
+    // simple cases. If another transaction wants to write the same row, it will
+    // wait until this transaction commits.
+    $stmt = $this->query('UPDATE {sequences} SET value = GREATEST(value, :existing_id) + 1', array(
+      ':existing_id' => $existing_id,
+    ));
+    if (!$stmt->rowCount()) {
+      $this->query('INSERT INTO {sequences} (value) VALUES (:existing_id + 1)', array(
+        ':existing_id' => $existing_id,
+      ));
+    }
+    // The transaction gets committed when the transaction object gets destroyed
+    // because it gets out of scope.
+    return $new_value;
+  }
 }
 
 /**