Column default values cannot contain braces on MSSQL

If you try to define a Field with a default value that contains a brace (for example a string literal 'a:{}') prefixTables() in Connection.php
will strip them out (or even prefix them!).

This is because the statement used to create and alter fields uses the DEFAULT literal (because there is no way PDO will let you bind parameters to an ALTER TABLE or CREATE TABLE statement):

    // $spec['default'] can be NULL, so we explicitly check for the key here.
    if (array_key_exists('default', $spec)) {
      $sql .= ' DEFAULT ' . $this->escapeDefaultValue($spec['default']);
    }

This is already fixed in the MSSQL server driver for 7.x, but this bug affects both 7.x and 8.x of drupal core.

Just reporting this if someone wants to fix it on the core database drivers. What I did on the MSSQL driver was to embed the table prefixing logic into the query generation, and then skip prefixTables().

Probably prefixTables() could be replaced by regex logic to overcome the issue.