diff --git a/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php b/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
index 91bbcb4cf6b6d134c2e48af6779c973517dda6bd..b9e8e731ab6bdc123e14ba6b0a0a7e8f9ce5e2e4 100644
--- a/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
+++ b/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
@@ -24,8 +24,11 @@
  * - 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.