Docs: Add important note on using iterall and iterdict (aiidateam…

…#6126)

Using the `all` and `dict` equivalents are very inefficient for large
query results and will lead to performance problems.

docs/source/howto/query.rst

@@ -74,12 +74,15 @@ There are several ways to obtain data from a query:
 
     all_results_l = qb.all()            # Returns a list of lists
 
-In case you are working with a large dataset, you can also return your query as a generator:
+.. tip::
+
+    If your query only has a single projection, use ``flat=True`` in the ``first`` and ``all`` methods to return a single value or a flat list, respectively.
+
+You can also return your query as a generator:
 
 .. code-block:: python
 
     all_res_d_gen = qb.iterdict()       # Return a generator of dictionaries
-                                        # of all results
     all_res_l_gen = qb.iterall()        # Returns a generator of lists
 
 This will retrieve the data in batches, and you can start working with the data before the query has completely finished.
@@ -90,6 +93,12 @@ For example, you can iterate over the results of your query in a for loop:
     for entry in qb.iterall():
         # do something with a single entry in the query result
 
+.. important::
+
+    When looping over the result of a query, use the ``iterall`` (or ``iterdict``) generator instead of ``all`` (or ``dict``).
+    This avoids loading the entire query result into memory, and it also delays committing changes made to AiiDA objects inside the loop until the end of the loop is reached.
+
+
 .. _how-to:query:filters:
 
 Filters