* - target: (optional) The database target name. Defaults to 'default'.
* - batch_size: (optional) Number of records to fetch from the database during
* each batch. If omitted, all records are fetched in a single query.
- * - ignore_map: (optional) Source data is joined to the map table by default.
- * If set to TRUE, the map table will not be joined.
+ * - ignore_map: (optional) Source data is joined to the map table by default to
+ * improve migration performance. If set to TRUE, the map table will not be
+ * joined. Using expressions in the query may result in column aliases in the
+ * JOIN clause which would be invalid SQL. If you run into this, set
+ * ignore_map to TRUE.
*
* For other optional configuration keys inherited from the parent class, refer
* to \Drupal\migrate\Plugin\migrate\source\SourcePluginBase.