function DatabaseConnection_pgsql::nextId

Retrieve the next id in a sequence.

PostgreSQL has built in sequences. We'll use these instead of inserting and updating a sequences table.

File

includes/database/pgsql/database.inc, line 184

Class

DatabaseConnection_pgsql

Code

public function nextId($existing = 0) {
  // Retrieve 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 processes
  // 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));
  // Retrieve 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;
}